Additional changes to Results based on phone discussion.

* Moved Units for X, Y & Z into ResultValueCoverages, in the same table
as their respective values.
* Removed CoverageTypeCV (and table), because that information should
be included in ResultTypeCV.
* Made many fields in ResultValues nullable, to accommodate PetDB (and
many other use cases) where this information is not known.
* Updated field and diagram comments in Results schema accordingly