Improve docs, unit tests, use one argument for radio_line distance spec.

* Add documentation of new equivalencies.
* Document the mass-energy equivalency.
* All unit tests now pass.  Add test for direct luminosity distance
  specification.
* Rather than having lumdist and cosmology as seperate arguments,
  just use one argument then try/fail to figure out what it is.
  So, instead of radio_line(freq, z, None, WMAP9), you now just
  use radio_line(freq, z, WMAP9) and the equivalency figures it all out.