Further performance improvements for earth-centred transformations

#11069 sped up the calculation of GCRS obsgeoloc and obsgeovel from EarthLocation for the GCRS<->(CIRS, TETE) transforms. It may be possible to

• Use the same technique to (slightly?) speed up erfa_astrom.apco (see https://github.com/astropy/astropy/pull/10994#issuecomment-730408512)
• Speed up EarthLocation.get_gcrs(). (see #11073)
• Be cleverer in the transformations, not using FunctionTransformWithFiniteDifference when all that is being done is introduce a rotating frame. EDIT: this would work only if obstime were the same; can one special-case this? EDIT 2: best to just special-case “interesting” transformations like ICRS->AltAz, as suggested in #10887.

[mkbrewer]

While the coordinate transforms don’t currently deal with the target’s velocity, they could be upgraded to do so. For instance, the ICRS->GCRS transform could deal with velocity in the same way that it does for position. The ErfaAstrom method apcs() returns the observer’s velocity wrt the SSB in astrom['v'] (as does apco()). This can be subtracted from the ICRS target velocity in the same way as astrom['eb'] is subtracted from the position to form the GCRS velocity of the target.

[mhvk]

@mkbrewer - belatedly, not super familiar myself with the new SpectralCoord class capabilities, so let me ping @astrofrog about your https://github.com/astropy/astropy/issues/11072#issuecomment-735241792

My guess is that it is possible - if so, it sounds like it would be good to have an actual example in the docs!