Beam_dict requires initializing all antennas in the MS even after after down selecting to one baseline.

[nmahesh1412]

In pyuvsim.uvsim.run_uvdata_uvsim the pyuvdata object passed has only one baseline. But the beam_dict parameter requires all the antennas in the MS set to be mapped to a beam_id in the beam_list. This is not the case when one passes beam_dict = None.