Allow for segments of branched entities to be defined and used in related categories

Similar to `ihm_entity_poly_segment`, a category can be created to identify segments of branched entities, pointing to either `pdbx_entity_branch_list` or `pdbx_branch_scheme`. 

The branch segment ids can then be an optional data item in other related tables such as `ihm_model_representation_details`, `ihm_struct_assembly_details`, and `ihm_starting_model_details`. 