Future enhancements to migration

[artgoldberg]

Listed in decreasing order of my subjective assessment of importance

0. avoid need for obj_model.core.ModelMeta.CHECK_SAME_RELATED_ATTRIBUTE_NAME by comparing related models by value, not name
1. use Model.revision to label git commit of wc_lang and automatically migrate models to current schema and report inconsistency between a schema and model file
2. move generate_wc_lang_migrator() to wc_lang
3. provide a well-documented example
4. YAML config examples with multiple existing_files and multiple migrated_files
5. use deepcopy on obj_model.ontology.OntologyAttribute attributes when deepcopy of pronto terms works
6. associate schema pairs with renaming maps
7. separately specified default value for attribute
8. improve performance of test_migration
9. obtain sort order of sheets in existing model file and replicate in migrated model file
10. confirm migration works for json, etc.
11. test sym links in Migrator.parse_module_path
12. use PARSED_EXPR everywhere applicable

[jonrkarr]

generate_wc_lang_migrator should be moved to wc_lang