pwkit/ucd_physics.py: import some dMe/UCD physics routines

I've significantly reworked them to be broadcasting-friendly
which might have been a bad idea. Oh well.

It looks like there was a bug in the tau_c computation used in chelsea.

Some of these need more work. For instance, the tau_c equations have
discontinuities that should be smoothed out since the linear fits are very
approximate anyway.