Add DataQuality table to ODM2Core

* along with DataQuality cross-ref table
* adjusted ODM2Core diagram
Create new ODM2DataQuality schema w/ 2 tables

* add ReferenceMaterials and NormalizationReferenceMaterialValues
tables to it, along with their relationships
* Add NormalizationReferenceMaterialValuesID to Results table
* Created ODM2DataQuality diagram
Add cross-ref to ExternalIdentifiers

Add ReferenceMaterialExternalIdentifiers cross-ref table in
ExternalIdentifiers schema
Adjusted Core and Overview diagrams

to accommodate NormalizationReferenceMaterialValueID in as a field name
in Results.
* Added Core.DataQuality and Core.ResultsDataQuality tables to Overview
diagram
Minor changes to DataQuality entities

I did the following:
1.  Changed DataQualityValueUnitID to DataQualityValueUnitsID for
consistency
2. Renamed relationships to the ReferenceMaterials entity, which still
had reference to “calibration”
Adding documentation page for the DataQuality extension

This page needs to be fleshed out with examples so we make sure we have
this right before merging it back into the master branch.  Right now
the documentation page exists in this feature branch only.
Update DataQuality

Changes as discussed in my comment today under
https://github.com/UCHIC/ODM2/pull/23
* moved DataQuality and ResultsDataQuality tables from ODM2Core to
ODM2DataQuality schema
* Changed DataQualityCodeCV to DataQualityCode and made it not nullable
* Removed NormalizationReferenceMaterialValuesID as a foreign key in
Results and implemented a cross-ref table with a 1:1 relationship to
Results.
* Changed: ReferenceMaterials is now directly linked to
SamplingFeatures rather than being a child of Specimens.
Started some documentation for Data Quality extension.

I started writing some documentation for the data quality extension.
It needs more work and examples.
DataQuality documentation update & field name changes

* expansion of documentation for optional DataQuality schema and tables.
* Changed NormalizationReferenceMaterialValues entity/table name to
ReferenceMaterialValues (same for it’s primary key), not only to
shorten it but also because it implies an overly narrow use for these
values.   However, the foreign key in Results was only slightly renamed
to NormalizedByReferenceMaterialValueID to make it’s intended use
explicit.
* added Comments to tables and fields in the DBwrench schema file
Added sensor calibration use case to ODM2 docs.

in Documentation
Update ext_dataquality.md
Clarified text in ReferenceMaterials documentation
Update ext_dataquality.md
Update ext_dataquality.md
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.
Update ext_dataquality.md regarding LIMS

Clarified statement regarding ODM2 and Laboratory Information
Management Systems.
Updates to DataQuality extension

1. Modifed relationship between SamplingFeatures and ReferenceMaterials
to be 1 to 0:1.  Thought I had done this before.  Not sure what
happened.
2.  Removed the comment from the DataQuality schema diagram. We now
have some nice documentation and so it isn’t needed.
3. A few modifications and additions to the documentation for the
DataQuality extension.
Merge pull request #23 from UCHIC/DataQuality_FeatureBranch2

Merging DataQuality_FeatureBranch2 back into the Master branch.