Update ext_dataquality.md and ODM2_DBWrench_Schema.xml

* Updates to documentation directly address questions brought up by
Emilio in his last commits.
* Added to documentation example use of ReferenceMaterialValues for
describing oxygen stable isotope Results.
* in ..Scema.xml, changed ReferenceMaterialTypeCV to
ReferenceMaterialMediumCV, which should pull from the same CV as
SpecimenMediumCV.  I think this is the primary information needed,
because the Variables are included in ReferenceMaterialValues.