Add more units for convenience

[zmitchell]

Currently you can only specify the units of an angle in degrees or radians. No one in the history of the universe has ever said "the angle is 1.5705 radians," but plenty of people have said "the angle is pi/2." To make describing angles more natural, it makes sense to allow the user to specify angles in units of pi. This is how that would look:

# defining an angle of pi/2
angle = 0.5
angle_units = "pi"

Another instance of this awkwardness is when you define the retardation of an optical retarder (phase plate). It would be natural to say that the retarder has a delay of some fraction of a wavelength, so adding a wavelengths unit would also be a quality of life improvement.