Merge SourceInfo feature branch into Master

These are all very straight forward additions that *I think* were well discussed and agreed on.  I've implemented these changes identically to what we fleshed out last week and shown here:
http://uchic.github.io/ODM2/schemas/ODM2_Feb26_MeetingBranch/diagrams/ODM2SamplingFeatures.html
http://uchic.github.io/ODM2/schemas/ODM2_Feb26_MeetingBranch/diagrams/ODM2ExternalIdentifers.html

Note that the Citations table is a placeholder for a near-future revision to implement Citations (or Attributions) in a manner similar to IEDA's draft  "data_source" and "author_list" tables in "PetDBSchema-Redesign-26.png".  This will require a bit of discussion and we'll do this later this week when we tackle the Provenance schema.