jjmccollum/open-cbgm: Improved Collation Parsing

This release ensures cross-compatibility with TEI XML inputs to teiphy with the following new features:

• The apparatus and variation_unit class constructors support an ignored_suffixes input to specify suffixes to witness sigla (e.g., "*" and "T" for first hands and "V" and "f" for witnesses with reconstructed and defective readings).
• The variation_unit class now includes a get_base_siglum witness that strips ignored suffixes from witness sigla encountered in the collation in an intelligent way (so that, for instance, the witness whose base siglum is "MT" won't have the "T" suffix stripped from it).
• The variation_unit constructor now parses both rdg elements (for proper variant readings) and witDetail elements (for ambiguities, overlaps, and lacunae).