Support complex angles

[KronosTheLate]

From the release announcement:
"Another problem is that you should support complex angles. For example, if you have a total-internal-reflected ray, the transmitted angle is complex, but it is still perfectly valid, and the complex reflection coefficient is still useful in telling you the reflected phase. You’ll also a complex transmitted “angle” if the transmission medium is lossy (complex index)."

I have never touched complex angles, but help on this is very welcome!
One thing that I would require of the implementation is that it still allows checking of the angle magnitudes (?), as angles above 90 degrees give silent errors (garbage results).

[KronosTheLate]

@stevengj Would you know how to implement this?