Move hyperparameters to model_definition instead of state object

Currently hyperparameters are specified as inputs to the [state constructor](https://github.com/datamicroscopes/lda/blob/d66e2d89ce2fbb1013038c661faa6e63c1a01412/microscopes/lda/_model.pyx#L29). Allow them to be instead specified in the [model_definition](https://github.com/datamicroscopes/lda/blob/d66e2d89ce2fbb1013038c661faa6e63c1a01412/microscopes/lda/definition.pyx#L3) constructor.
