Adding support for future array shapes in UVBeam objects

[kartographer]

Following the merge of #909, UVBeam objects and methods now need to be updated to allow for the new handling of multiple spectral windows, and more generally to allow for the proper handling of the flexible spectral window feature found in UVData.

[bhazelton]

I’m working on future_array_shapes and flex_spws for UVBeam. I think I’ve got all the future array shapes parts worked out, so I started thinking about flexible spws. The more I think about it, the less clear it is to me that it is a useful concept on UVBeam.

The UVBeam object has a freq_array, but it doesn’t have a channel_width because that’s not a very sensible concept for simulations (or measurements) of beam responses — they’re generally just at a particular frequency. One of the most widely used methods on UVBeam is interpolation (both in frequency and spatially). When I started trying to think about how interpolation in frequency should interact with spectral windows I realized that I don't really know that spectral windows is a useful concept on UVBeam.

I'd love to hear thoughts about what utility spectral windows provides for UVBeam objects.

[bhazelton]

A suggestion from @kartographer is to not implement any flexible_spw related code but also just make the existing spw_array and Nspws parameters optional rather than removing them in case we wanted to implement something similar to the wide_band mode of UVCal in the future to describe beam models that are valid over a range of frequencies.