New xtypes for sexagesimal angles

[mbtaylor]

There is currently no good way to indicate that a VOTable FIELD/PARAM contains a sexagesimal representation of an angle. Common practice in VizieR is to use a value of 'h:m:s' or 'd:m:s', but these are not units. Client code might infer that string values with UCDs indicating celestial coordinates are sexagesimal, but it's a bit hacky.

I would like to see two new xtypes in DALI to indicate string values that represent angles in sexagesimal hours:minutes:seconds and degrees:minutes:seconds. Presumably these would be xtypes "hms" and "dms".

[pdowler]

Agree.

[pdowler]

I'm working on a PR with xtype="range" and removing multipolygon from shape, so will add these at the same time.

[pdowler]

this is in PR #24