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.