c_term_coverage: add a few more system and OID nicknames

[mikix]

This adds CPT, ICD9CM, ICD10CM, and UCUM systems to the metric's short names, as well as OID versions of each system.

With this change, for example:

• "http://unitsofmeasure.org" -> "UCUM"
• "urn:oid:2.16.840.1.113883.6.8" -> "UCUM (OID)"

Plus a little bit of system string refactoring to avoid repeating strings that are easy to typo.