Add a function to generate component masses

The function is analogous to generate_mass_parameters but does the conversion from chirp mass and mass ratio to mass 1 and mass 2 (i.e. the reverse conversion) The conversion requires computing the total mass, it can also be saved by setting a flag. (This is designed to be merely a convenience.)

The analogous functions seem to be untested, but I'm happy to add a test I've written to the conversion tests if it'd be helpful.