Equivalencies for Radio line strength units, like K km/s pc^2 and Jy km/s

Add equivalency pairs for radio astronomy line strength units.  There is one simple conversion (Jy km/s to W/m^2) that only requires the observing frequency, the others require more information (the redshift, frequency, and luminosity distance).  Therefore provide two equivalency classes: one (`radio_lines_simple`) that can only do the first conversion, the other (`radio_lines`) that can handle the others, but at the price of requiring more information.

Documentation and unit tests included.