Add supporting code for selecting representative CSMs (for multi-model systems)

[piehld]

Add supporting code for selecting representative CSMs (for multi-model systems)

@piehld https://github.com/rcsb/py-rcsb_utils_dictionary/blob/ro-4455/rcsb/utils/dictionary/DictMethodCommonUtils.py#L4589 and https://github.com/rcsb/py-rcsb_utils_dictionary/blob/ro-4455/rcsb/utils/dictionary/DictMethodCommonUtils.py#L4506 do not handle representative models for computed structure models. See https://mmcif.wwpdb.org/dictionaries/mmcif_ma.dic/Categories/ma_model_representative.html.

Originally posted by @brindakv in #84 (comment)

Also see:

• V1.30 Only load instance features for the representative model #84 (comment)
• V1.30 Only load instance features for the representative model #84 (comment)