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.