Rac-schemas

Adds
- Adds valid date type of `range` to enum.

Adds
- Allows null on rights statement end dates.

Adds
- adds required `group` field on Collections, Objects, Terms and Agents.

Adds
- Removes minNumber on extent to allow for an empty array.

Adds
- Makes `begin` fields on the `date` object optional (which they were before, although in a somewhat obscure manner, due to a missing `type` field).

Fixes
- Makes sure date instance is a string before trying to parse.