Allow passing a UVBeam readable file to `beam-file-path`

[bhazelton]

I was very confused that this didn't work. I finally figured out (by reading the code) that I had to put the MWA data file in the repo under PyFHD/PyFHD/resources/instrument_config/. I don't like this approach because it mixes data and code and because it's undocumented. I think it's much more straightforward to pass it via the beam-file-path keyword, but that appears to only accept some sort of undocumented beam format, not UVBeam formats.