Type hints for derived quantities

[dlakaplan]

Using the older syntax for combining type hints and quantity input since we still support old astropy (#1709)

[dlakaplan]

I had to change the docstrings to raw docstrings (r""" ... """) to avoid stricter checking for escape sequences in python 3.12.

[dlakaplan]

This is broadly failing the oldestdeps. I haven't tracked it down, but it might be something we table until we update the oldest astropy.

[dlakaplan]

So I think the issue with the older astropy is a conflict between the quantities decorator and typehints decorator for the return types. e.g.,

@u.quantity_input(x=u.m)
def v(x: u.Quantity) -> u.Quantity
    return x/(10*u.s)

seems to fail with the older astropy since it interprets the return type for a return unit. Without the @u.quantity_input this works fine, and in newer astropy it works. But not with the old ones. Instead we need to do:

@u.quantity_input(x=u.m)
def v(x: u.Quantity) -> u.m/u.s
    return x/(10*u.s)

and specify the units for the return rather than the type/class. Probably this is OK and combining the unit checking with type hints is still a net positive, but we might want to check back on this when we update oldestdeps to see if we can make it more uniform.