Add Artifact description lookup

[dr-rodriguez]

We think it would be useful to have an element in the Artifact section that perhaps stores a URI that serves as a lookup for description information. MAST uses an internal ID to point to a table with descriptions for each type of file, which we pull for our interfaces. Storing this in the CAOM model itself can allow us to expand that so that we can share file descriptions across archives.

As an example, our table contains entries like these (plus internal IDs and other information):

DADS C3T file - Calibrated data WFPC2
exposure (L2c): 2D Calibrated data averaged over integrations
Calibrated full frame image