Add attribute to record secondary forecast sources when clustering realizations

[gavinevans]

Related to metoppv/improver#2332

The only significant change introduced in this PR is an additional attribute "secondary_input_realizations_to_clusters", as can be seen in the print out below.

Processing  single_lead_time_kgo.nc
different attributes of /lwe_precipitation_rate - ['secondary_input_realizations_to_clusters']

lwe_precipitation_rate / (m s-1)                 (realization: 2; projection_y_coordinate: 200; projection_x_coordinate: 200)
    Dimension coordinates:
        realization                                          x                           -                             -
        projection_y_coordinate                              -                           x                             -
        projection_x_coordinate                              -                           -                             x
    Scalar coordinates:
        forecast_period                          21600 seconds
        forecast_reference_time                  2025-12-01 06:00:00
        time                                     2025-12-01 12:00:00
    Attributes:
        Conventions                              'CF-1.7'
        cluster_sources                          '{"0": {"uk_ens": [21600]}, "1": {"uk_ens": [21600]}}'
        history                                  '2025-12-01T07:53:52Z: StaGE Decoupler'
        institution                              'Met Office'
        primary_input_realizations_to_clusters   '{"0": [19], "1": [0, 18]}'
        secondary_input_realizations_to_clusters '{"uk_ens": {"0": [{"realization": 5, "forecast_periods": [21600]}], ...'
        source                                   'Met Office Unified Model'
        title                                    'MOGREPS-UK Model Forecast on UK 2 km Standard Grid'
lwe_precipitation_rate / (m s-1)               (realization: 2; projection_y_coordinate: 200; projection_x_coordinate: 200)
    Dimension coordinates:
        realization                                        x                           -                             -
        projection_y_coordinate                            -                           x                             -
        projection_x_coordinate                            -                           -                             x
    Scalar coordinates:
        forecast_period                        21600 seconds
        forecast_reference_time                2025-12-01 06:00:00
        time                                   2025-12-01 12:00:00
    Attributes:
        Conventions                            'CF-1.7'
        cluster_sources                        '{"0": {"uk_ens": [21600]}, "1": {"uk_ens": [21600]}}'
        history                                '2025-12-01T07:53:52Z: StaGE Decoupler'
        institution                            'Met Office'
        primary_input_realizations_to_clusters '{"0": [19], "1": [0, 18]}'
        source                                 'Met Office Unified Model'
        title                                  'MOGREPS-UK Model Forecast on UK 2 km Standard Grid'

Processing  multiple_lead_time_kgo.nc
different attributes of /lwe_precipitation_rate - ['secondary_input_realizations_to_clusters']
lwe_precipitation_rate / (m s-1)                 (realization: 2; time: 2; projection_y_coordinate: 200; projection_x_coordinate: 200)
    Dimension coordinates:
        realization                                          x        -                           -                             -
        time                                                 -        x                           -                             -
        projection_y_coordinate                              -        -                           x                             -
        projection_x_coordinate                              -        -                           -                             x
    Auxiliary coordinates:
        forecast_period                                      -        x                           -                             -
    Scalar coordinates:
        forecast_reference_time                  2025-12-01 06:00:00
    Attributes:
        Conventions                              'CF-1.7'
        cluster_sources                          '{"0": {"gl_ens": [43200], "uk_ens": [21600]}, "1": {"gl_ens": [43200], ...'
        institution                              'Met Office'
        primary_input_realizations_to_clusters   '{"0": [19], "1": [0, 18]}'
        secondary_input_realizations_to_clusters '{"uk_ens": {"0": [{"realization": 5, "forecast_periods": [21600]}], ...'
        source                                   'Met Office Unified Model'
lwe_precipitation_rate / (m s-1)               (realization: 2; time: 2; projection_y_coordinate: 200; projection_x_coordinate: 200)
    Dimension coordinates:
        realization                                        x        -                           -                             -
        time                                               -        x                           -                             -
        projection_y_coordinate                            -        -                           x                             -
        projection_x_coordinate                            -        -                           -                             x
    Auxiliary coordinates:
        forecast_period                                    -        x                           -                             -
    Scalar coordinates:
        forecast_reference_time                2025-12-01 06:00:00
    Attributes:
        Conventions                            'CF-1.7'
        cluster_sources                        '{"0": {"gl_ens": [43200], "uk_ens": [21600]}, "1": {"gl_ens": [43200], ...'
        institution                            'Met Office'
        primary_input_realizations_to_clusters '{"0": [19], "1": [0, 18]}'
        source                                 'Met Office Unified Model'

[maxwhitemet]

Happy with the changes made. Approved 👍