Small issues with Beam

[mhvk]

As I was looking at the beam code for astropy/astropy#17096, I noticed a few oddities in Beam. Mostly that it inherits from Quantity, which I'm not sure is a good idea given that some basic methods are overwritten and many others would not work (e.g., you'd need a proper __array_finalize__ to deal with slicing, etc.; right now, you'd lose the extra attributes with you do beam[()]). If there are real advantages to using Beam, I'd recommend using SpecificTypeQuantity.

A second thing I happened to notice is that default_unit is not actually used as described in the docstring.

Anyway, just flyby comments - hopefully of some use.

[e-koch]

Thanks @mhvk ! I opened #128 to fix the specific issue in astropy/astropy#17096, and the default_unit description and usage.

agreed, we should look into switching to SpecificTypeQuantity. I don't think we otherwise have a specific need to inherit from Quantity