AntennaXds: Additional mount types

[kettenis]

PR casacore/casacore#1386 is going to introduce two additional mount types for telescopes with an ALT-AZ mount that use a Beam Wave Guide to bring the focus down to the pedestal. Compared to the already supported Nasmyth mounts there is an extra correction term because there are now two rotating mirrors. See https://arxiv.org/abs/2210.13381 for more details. There are two possible geometries and analogous to the Nasmyth mount types these will be called ALT-AZ+BWG-R and ALT-AZ+BWG-L.

[FedeMPouzols]

This kind of addition would be a documentation update in the schema file, as we currently have the mount defined as a string, and the "Reserved keywords" simply listed in its docstring:

    mount: Coord[AntennaName, str]                                                                                                                                                                                                                            
    """ Mount type of the antenna. Reserved keywords include: ”EQUATORIAL” - equatorial mount;                                                                                                                                                                
    ”ALT-AZ” - azimuth-elevation mount;                                                                                                                                                                                                                       
    "ALT-AZ+ROTATOR"  alt-az mount with feed rotator; introduced for ASKAP dishes;                                                                                                                                                                            
    "ALT-AZ+NASMYTH-R": Nasmyth mount with receivers at the right-hand side of the cabin. Many high-frequency antennas used for VLBI have such a mount typel;                                                                                                 
    ...

This would apply to the AntennaXds and also GainCurveXds and PhaseCalibrationXds.

[kettenis]

Thanks @FedeMPouzols, I'll create a pull request for that once the casacore pull request goes in.

[FedeMPouzols]

The branch of this issue has the addition that I think is needed: #377.

As a side minor issue, we have duplicated docstrings in a number of places. This antenna_mount is a remarkable one. Some projects use different tricks to avoid text duplication, such as inheritance or/and decorators or similar, but as that requires execution of code it might not work well in all parsers / tools / IDEs. Another alternative would be to just refer to the "main" antenna_mount of AntennaXds with a sphinx link, but that adds a level of indirection that may make it more difficult to quickly read through the docs of one xds.

For antenna_mount, if we added a list of possible values / reserved keywords at the end (similar to the list of intents) we could refer to that list and remove the lists in every mount docstring, but similarly it is probably not as comfortable to read (when the list is not too long such as the intents list).

Related, but independently of that, there could be lists of possible values / reserved keywords at the end for the processor type (CORRELATOR, SPECTROMETER, RADIOMETER), and gain curve type. I think those are the other lists that we have. These two last ones probably never get additions?
I'd tend to favor this approach of having all the lists of reserved keywords at the end, so that it is very explicit which fields/attributes have such reserved keywords.