Make ncobs path configurable

[maddenp-cu]

wxvx configures pb2nc in such a way that the netCDF files it creates may be appropriate to one verification config but not another. For example, it may set a masking grid based on the forecast-grid mask, filtering many observations from the raw prepbufr. If a verification config configured to use a difference forecast-grid mast then uses these .nc files, it may not have access to the correct set of observations. That is, the raw .nr prepbufr files will be appropriate to any point_stat-based verification config, but the .nc files created by pb2nc will only be appropriate to certain verification config.

Currently, the .nc files are created in the same directory as their .nr counterparts, so to get a different set of .nc files for a different verification config, we would have to change the paths.obs value in the wxvx config. But that would mean downloading all the .nr files again to this diffrerent directory -- a waste of time and space.

So, we should be able to specify separate paths for .nr and .nc files so that the .nr files can be reused and different sets of .nc files can be created for different verification configs.

[maddenp-cu]

Could be subsumed in work on #118.