less chatty on model fitting; doc. updates; category filter update

- cfilter -> category_filter,
- category_filter now takes functions that return either logical
vectors or indices
- category_filter reversed to align with the name 'filter'; ie, we keep
the categories matching the criteria, rather than remove them
- return the data used as part of the model fitting output
- set class of model output object in entirety, rather than just the fit