New guidance needed for units in EML

[mobb]

currently, units are mentioned in several places, e.g., under <additionalMetadata> and <attributeList>.
but some basic bits of info are missing, or hard to find. e.g., these use cases:

1. Is unit XXX on the standard list? it makes EML construction easier if you can pick from a list. And the list is significantly cleaned up and broader.
2. How is customUnit XML constructed? It's here, but Jon Ide didn't find it when coding for ezEML.

We have not yet decided on a plan for the unit registry (from LTER). see that repo: https://github.com/EDIorg/unit-registry

Until we do, here is a temporary list of the EML 2.2 std units, just the non-deprecated ones.
https://sbclter.msi.ucsb.edu/external/InformationManagement/EDI/units/EML_units_preferred.html
it could have a home in the EML metadata chapter.

Unit chapter should also have info about the EML 2.2 unit features (the clean-up act, deprecated irregular spellings) and UDUnits synonyms
and link to this #7

[gremau]

Revising and keeping open. We will need to write a guide to using the new QUDT system.