Update scripts for creating feathered, weighted cube mosaic

[d-l-walker]

This is sort of an updated combination of #361 and #365.

Thanks to @ashleythomasbarnes, the master script is much more concise now, and pulls in functions from separate scripts. It now also uses CASA tasks for the regridding to a template (similar to @pyhsiehATalma's approach), which is a lot faster than our previous versions.

I've added functionality to crop and regrid the cubes in velocity prior to mosaicking, as well as the ability to include/exclude 12m data.

The main bottleneck in terms of runtime is the cube cropping and regridding. We can look into the possibility of speeding this step up.

The code still needs more robust error handling, but for now it should work well provided you have the proper directory set up.

[There will be a bunch of style issues that I will fix before merging ...]

[d-l-walker]

Done.
TODO: Update relevant function(s) to use radio_beam.Beams.common_beam when smoothing the data to a single resolution. This will be more robust than the current approach.

[keflavich]

What is the return () for at the end of each? If that's not needed, I'd recommend removing it.