Move RA.hour_angle and RA.lst methods to standalone functions
Remove RA and Dec subclasses
Remove useradec and anglebounds from _initialize_latlon()

These two args are no longer needed in the simplified system of
just having Longitude / Latitude classes that function as
RA / Dec, Az / Alt, and L / B respectively.

One notable API change is that Galactic "l" is now in 0 to 360 instead of
-180 to 180.  This required a test change.  From what I can see in
the literature Galactic "l" should be 0 to 360.