Create some kind of configuration file for various hard-coded checks

[eckardm]

Describe the bug

Various hard-coded checks, especially for parsing items, could be more flexibly implemented in a configuration file. Again, minor thing, probably. See, for example:

• this dictionary of expected file formats and extensions;
• the minimum number of required files and maximum number of files permissible;
• filenames we don't care about;
• the regex for digital part names; and
• the number of expected files (1 or n, for example) for "item" files.

Those are just some ideas.

Expected behavior

If these could be configured by an administrator, that might help if our specifications change.

Additional context

See our notes from the Sprint #1 check-in on September 17, 2019.

[eckardm]

This might be especially useful if/when BAroQUe handles video, as the METS validation is different for audio and video (so some way to configure it would be good).