Investigate IVOA xtype=timestamp and, if appropriate, apply it to all Felis timestamp types

The DALI rec indicates that timestamps should have this combination of attributes:

datatype="char"

arraysize="*"

xtype="timestamp"

We already implement the first two in the Felis TAP interface. The xtype can be easily added.

The text also indicates that “the arraysize may be set to a more specific value if it is known,” which we do not calculate. All timestamps are hardcoded to have arraysize="*" if it is not explicitly provided by the arraysize field on the column (which none of the timestamp columns in the existing schemas currently do).