Simulations require full polarization

[rlbyrne]

I am trying to simulate data in XX and YY only, but pyuvsim currently only supports Npols=4.

[rlbyrne]

This is actually related to using multithreading, and specifically to using the mpi.big_bcast command. Something is getting lost when the uv object is broadcast across threads in that way. I need to look into this further.

[bhazelton]

We recently merged #394 which adds support for single feeds. That would allow you to simulate XX and YY separately. Note quite what you're asking for, but might be of interest.