Add SCDLCharlestonMapper for object mapping (#7651)

SCDLCharlestonMapper extends SCDLMapper in order to perform special
mapping of the 'object' property that is different from that of
the other SCDL providers.  The included test describes the desired
behavior.

Also, change Charleston "sets" property, since Charleston have
foregone sets with their new server, and this is necessary to
be able to fetch something that can be mapped.
Merge pull request #7 from dpla/7651-update-charleston

Add SCDLCharlestonMapper for object mapping (#7651)
UTC timestamps everywhere; fix sitemaps (ref #7657)

* `ingestDate` and dashboard doc timestamps are now in UTC with timezone
* Sitemaps require `<lastmod>` dates to be in W3C Datetime format
* Updates JSON Schema to check if `ingestDate` is RFC 3339 compliant
* Use `ingestDate` from items to populate `<lastmod>`
* Move configuration loading logic outside functions
* Add configuration option for local sitemap path

Note: RFC 3339 is a subset of W3C Datetime/ISO 8601.
Merge pull request #6 from dpla/7657-fix-sitemap-generation

Use UTC timestamps everywhere/improve sitemap generation (fixes #7657)
Bump version to 32.0.0
Adding docstrings to explain shred module (#7613)

A minor refactor for more sensible variable names is included.