Merge Provenance feature branch

I've made a number of changes that significantly clean up the Provenance schema.  It had contained several legacy tables that were put there prior to a number of other enhancements in other schema that now made them redundant.  We had not discussed most of these changes previously, so please review and let us know if these changes make sense.

In the end I did not make the Citations match the "data_source" table from PetDBSchema-Redesign-26.png because many of the field names were very specific to journal articles.  However, I did match the overall structure.

My only additional suggested change here is that I might prefer the term Attributions over Citations, because the former term is a bit more general and does not imply that something has formally been published.  I am also wondering if the Citations (or Attributions) table should be moved from Core to Provenance schemas. That is where it belongs conceptually, but I'm not sure if it needs to be required or not for all use cases.  Furthermore, if it becomes optional, then we have to remove direct references to it from within all required tables, which adds another small layer of complexity.

Merging this feature branch would close #8.