translate custom and QUDT unit metadata to eml
get_custom_units.Rdget_custom_units harvests metadata pertaining to units
of type custom and QUDT detailed in `custom_units.yaml`. Custom units, i.e.,
those not documented by QUDT or included in the EML standard libary, have a
id attribute and description element, where as QUDT units have only a id
attribute since their presence in the EML <unitList> is merely a placeholder
to accommodate EML congruence.