support alternative representations of plane metadata

[pdowler]

The current plane metadata is roughly:
position: ICRS (deg)
energy: (barycentric) wavelength (m)
time: MJD UTC (d)
polarization: list of states

These coordinate/reference systems and units are now listed in the "interoperable profile".
However, the CADC TAP service also provides some energy columns with frequency values because the typical query cannot be re-written in a way that can be (easily) indexed...

In principle, one could specify places to put values with alternate representations. This is most obvious for wavelength/frequency/energy/velocity, less so for position (icrs/other epochs/galactic?), and then maybe time frames....

[pdowler]

Thoiughts in no particular order:

• Is there a good use case for alternative profiles?
• Do they need to be defined and agreed upon or is this just value added content in a TAP service (the way CADC does it now)?
• If there were alternate profiles, could one harvest caom instances and perform a profile transformation or harvest partial instances (with one profile) and add the others to make a multi-profile service?

[pdowler]

I think the correct approach is:

• model defined with only non redundant fields
• the TAP relational mapping can specify alternate columns with well specified transformations from the model; these could be virtual or materialised (implementation detail).