Let Telescope objects be defined in a self-contained way in YAML

[steven-murray]

It would be nice if the telescope parameters were able to be defined in a single section of the configuration, e.g.:

telescope:
    name: hera
    antenna_numbers: ...
    etc.

It would be nice if this format could be baked into the Telescope object (in pyuvdata) so you could do Telescope.from_yaml(yamlfile), but it could also just be a pyuvsim function: telescope = read_telescope_yaml(yamlfile). We don't necessarily need to break the config format (we could still allow whatever keys were outside the telefile to exist outside, but also support them being inside the telescope: section).