Load class weights for non-CNN models

When I was refactoring for #230, I noticed that the class weights were only ever loaded for the CNN model. For the Hierarchical and CNLP models the class weights were always taken from the legacy dataset.class_weights which (as far as I can tell) was always `None`. To avoid changing functionality in the refactor I left these as `None` in the new code, but this is almost certainly a bug.