Allow relative path for uvbeam specification

[burdorfmitchell]

It would be useful to allow a relative beam_path specification for the uvbeam file in the simulation config yamls. We currently use the absolute path, which is understandable because the user is required to download the uvbeam file separately. A relative path would require less file editing for each user, and having an absolute path such as /users/dseitova/data/dseitova/HERA_NicCST_fullfreq.uvbeam as the default feels wrong (we have a better placeholder for the 2nd generation simulations but that one might also be deceptive).

[bhazelton]

@burdorfmitchell what should the path be relative to? The location of the obs param file?

[burdorfmitchell]

Relative to the teleconfig file seems like the clearest approach to me though I could see it going either way -- I believe someone writing a path in a teleconfig file would picture the path relative to that file. If that is weird to implement I think the obsparam file would be completely fine.

The reason for this issue was mostly to better handle the reference simulation config files (and small QOL as requiring the absolute path makes the config files a little harder to generalize and hand off to people)? The caching approach handled that.

Implementing this would be cool but this issue possibly lives a better life in pyuvdata and could be closed if that would help with issue cleanup.