REFACT: Manifest classes

[carlwilson]

• eark_validator/cli/app.py:
  • validation now uses eark_validator.model.ValidationReport module;
  • improved argparse docs a little;
  • updated epilog date;
• eark_validator/infopacks/manifest.py:
  • FileItem and Manifest types moved to appropriate model modules;
  • ManifestEntries and Manifests classes now hold API/factory methods for above;
  • no more Checksums from METS files;
  • uses Path rather than str for file paths;
• eark_validator/mets.py b/eark_validator/mets.py:
  • METS validation moved to MetsFiles class;
  • use FileItem rather than FileEntry types for file lists;
  • utility methods for FileEntry added;
• added file headers where missing;
• added a SchematronRuleset.get_reports() genrator method;
• improved type hints here and there;
• better/refactored tests; and
• fixed imports post-refactoring.

[carlwilson]

Subsumed by quick merge of a subsequent PR