Make angles units more consistently treated #173
Description
Currently, angles like degrees, radians, and steradians, are treated as a separate type of unit on somewhat the same footing as temperature, length, time, etc (though it is still considered dimensionless). This may pose a problem without more careful consideration. For example, if I have a rotation frequency in rpm or Hz but I want to convert it or compare it to an angular frequency which is in rad/s, I would need to multiply by 2*pi radians. In particular, rad/s and rpm are not considered equivalent units (see Dimensions.eq on line 114 of sasdata/quantities/_units_base.py). This may not be the desired behavior. Evaluate whether a better treatment is called for.