Made ResultValueCoverages a subclass of ResultValue

By moving X, Y, Z location information into a subclass table, with a
1:1 (or 1:0) relationship with Results, we decrease the complexity of
Results while not incurring a heavy join cost, because both tables
share a primary key.
* Created new ResultValueCoverages table.
  * Moved the following fields from ResultValues to this new table:
ValueX, ValueY, ValueZ, CoverageOriginID (these are slightly renamed).
  * Added new fields: CoverageTypeCV, CoverageLabel (for text coverage
info).
* Renamed OffsetOrigins to CoverageOrigins.  This table no longer links
to ResultsValues but rather to ResultValueCoverages.
  * Removed the associated Units field.
* Added CoverageTypeCV table to illustrate the information that would
specify all the required information relative to understand the
coverage dimensions.
  * NOTE that Coverage Dimensions do not need to be spatial.  This will
allow for spectra and other array types where the
ResultValues.DataValue all have the same Variable and Units (i.e. one
can not use this to store a typical data table as an array).