Add pix2ang functions

[cdeil]

@michitaro - How do you feel about adding pix2ang functions?

In my experience ang is commonly used, not just vec.

This is the equivalent of http://healpy.readthedocs.io/en/latest/generated/healpy.pixelfunc.pix2ang.html

If you want to include it, I could try to add a test.

[michitaro]

Thanks!