Add VOUnit reference

[gmantele]

A user noticed that the column TAP_SCHEMA.columns.unit as described in Sec. 4.3 of TAP-1.1 does not require or even does not mention the standard VOUnit. Should values of this column follows VOUnit or is it completely free?

[mbtaylor]

Since the units metadata is likely to make its way into output VOTables, it would make sense for both the TAP_SCHEMA.columns.unit column and the corresponding unit element defined in VODataService to follow the same rules as VOTable. The relevant text in VOTable has been in some flux, but following recent discussion (ivoa-std/VOTable#28), the current WD-VOTable-1.5 sec 4.4 says:

The syntax of the unit string SHOULD conform to the VOUnits specification

so I would say an addition of similar text in TAP (and VODataService) would be a good idea.

It's probably not fair to call the omission of such text in TAP-1.1 a bug, since REC-TAP-1.1 was published a little before REC-VOTable-1.4, which was the first VOTable version to mention VOUnits (VOTable 1.3 and earlier recommended use of the CDS catstd unit syntax).