Extend fftvis to allow per-antenna beams

[tyler-a-cox]

There is nothing in the current implementation of fftvis that would prevent us from simulating arrays with different beams, so we should probably make a PR to allow users to supply per-antenna beams. This doesn't scale very well the number of unique beams. We'd need to do a total of N_beams * (N_beams + 1) / 2 NUFFTs, which definitely does not scale as well as matvis, but still may be faster than matvis if the number of independent beams is small.

[steven-murray]

Totally agree. I think we could do a check in the setup and raise a warning if the number of beams is "large" to say that this is not efficient and a different simulator should be used.

[kartikmandar]

Do you want me to incorporate this? And should I add this to the GPU PR or after it is merged? Then, should I branch off from GPU one or the main? Let me know.

[tyler-a-cox]

No, we haven't included this in the CPU version just yet, so I think we can leave it out of the GPU version for now. When we do update the CPU version, we should probably also update the GPU version though.