Add Certification and test lab terms to Entity

[cwhanse]

Initial request is to record CertificationAgency and CertificationBody

Useful to track who did the testing that supports a certification? test labs, not much argument for specific data elements for test labs.

[cwhanse]

Larger questions that surfaced in discussion:

• when do we inherit Entity vs. using Contact?
• ought to be a user manual page listing named Entities (e.g., AHJ, etc.)

[cwhanse]

Closed by #305