refactor(beamform): refactor how primary beam model is evaluated (#131)
refactor(beamform): refactor how primary beam model is evaluated (#131)

- Add capability to track sources for a declination
dependent amount of time, so that all sources are
tracked through the same angle on the sky.

- Add option to ignore primary beam model when beamforming,
i.e., use uniform weighting.

- Use the beam method of the telescope object to evaluate
the primary beam model.  Requires the ss/tel-beam branch
of ch_pipeline for the CHIME implementation.

- Move the parameterized fit to the driftscan beam out of
this module and into its own telescope class in ch_pipeline.

- Check if a catalog is already in CIRS coordinates.

- Add the catalog's tag to the output container's tag.

- Create versions of the beamforming tasks that use an
external beam model provided at setup in a GridBeam container.
radiocosmology/draco,radiocosmology/draco