Initialize PolyBeam, PerturbedPolyBeam, and ZenikeBeam through obsparam

[piyanatk]

Currently this is not possible because the pyuvsim interface under the hood only know about AnalyticBeam. These beam can be used by pyuvsim, but keeping these beams here make the most sense because they are specific to HERA.

Some ideas for workaround

1. Open an issue in pyuvsim to allow "other" analytic beam type
2. Use an extra keyword in the obsparam to specify these beams. This keyword should be passed to the UVData object. We then reinitialize the beam.
3. Write our own obsparam initialization

[steven-murray]

I feel like the right way to do this is actually to fix the AnalyticBeam API. At the moment it is completely non-modular, and only allows you to specify specific strings to get different kinds of beams. It should define different beams as objects with a specific API that can be defined outside the original code. This will require talking to RASG -- I know there are plans to move the uvbeam stuff around.

Calling in @bhazelton and @dannyjacobs for comments here.