Add standard_name variable mappings to Requirements for GTS Ingest test?

[mwengren]

Placeholder issue for future discussion.

Building off of the test to be added in #759.

NDBC maintains a list of CF standard_name variables and associated mappings to variables they publish to the GTS. Some info is available here: https://github.com/ioos/compliance-checker/issues/748#issuecomment-597256009.

As an addition to the check being developed in #759 to cover dataset attribution rules for GTS ingest in the IOOS Metadata Profile, we could also check that each variable that meets those requirements has a standard_name known to NDBC.

One issue might be where to source the list from. Plans are to add information on accepted standard_names to the IOOS Metadata Profile docs, but if any changes are made in the future there would be at least two locations to keep updated (CC code, profile documentation).

cc @kbailey-noaa

[mwengren]

@Bobfrat @daltonkell We added the initial version of NDBC 'accepted' CF Standard Names to the IOOS Profile 1.2, see the XML to CF Standard Names section in the docs - PDF with details linked from there.

This would basically constrain the accepted CF Standard Names in #7 in the above list.

For a first version of this test, let's just add this initial constraint and flag/fail variables with any names that aren't in the list in the PDF.

We could enhance the test in the future to flag separately variables that have valid standard names but aren't in the list... but this will be a bit more involved/complicated I think.

cc @kbailey-noaa @jessicaaustin