Clean up variable names in cvmix_background

Finally replaced static_visc and static_diff with static_Mdiff and static_Tdiff
(should have been done long ago). I also started cleaning up the way we ensure
numbers are double precision - instead of some instances of 0.123_cvmix_r8 and
other instances of real(0.123,cvmix_r8) I'm trying to use real(*,cvmix_r8)
everywhere.