Add more header and column metadata for generating docs

[kosack]

Extra metadata fields would be useful for generating documentation:

• description for all elements
• reference or citation for documenting which standard defined the keyword/column (HDUs already have HDUCLASS/HDUDOC)
• unit or physical_type for cards (for docs, and maybe validation by string conversion). Note that the FITS standard suggests having the unit in square brackets in the comment string, i.e. EMAX = 1.2 / [TeV] The maximum energy, though it is not enforced. However, for VODF we could check for the existence of the correct unit in the doc and warn if not.
• ivoa_key reference/citation of where this key/column/hdu was defined
• ucd for defining Column UCDs (maybe also for Headers, only for documentation as there is no way to store it?)
• examples like in JSONSchema for headers at least (a list of example strings)