caom2-compute: WCS validator should check for restfrq|restwav if spectral axis is velocity

[pdowler]

restfrq and restwav are technically optional, but if the spectral ctype is velocity then one of them is needed or the validation fails in an obscure way.

The validator should check that one is provided when the axis is velocity and throw an exception with a good error mesage like "one of restfrq or restwav is required for axis with ctype={the ctype value}"