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).