RO-4895 add WF operations for ligand quality and residue rscc statistical reference calculations

[shaochenghua]

Added two modules under py-rcsb_workflow/rcsb/workflow/stats, and their execution operations in exdb_wf_cli.
Expected outputs are two files in CACHE:
"ligand_score_reference.csv" for ligand quality reference from --op "ligand_quality_ref_gen";
"rscc-thresholds.json" for residue RSCC reference from --op "residue_rscc_ref_gen".

Local testing command example:
exdb_wf_cli --op ligand_quality_ref_gen --config_path /Users/chenghua/Projects/RCSB_quality_reference/test_run/py-rcsb_workflow/rcsb/mock-data/config/dbload-setup-example.yml --config_name site_info_configuration --cache_path .

To make the above command work, I had to tweak the current mock-data/config/dbload-setup-example.yml to add MONGO_DB_URI.

Also, the two test files of "testLigandQualityReferenceGenerator.py" and "testResidueRsccReferenceGenerator.py" may need your update in their setUp() method to make them work with the mock data, i.e. please pay attention to configPath and defaultSectionName setting. Once the setup is proper, no further update is needed to make the unit tests work.