Fix for issue GFDL is having with coeffs_kpp_low

I renamed compute_nu_OBL_depth to compute_nu_OBL_depth_LMD94 and removed the
branch of the routine that handled the case where we wanted to use an
interpolation scheme different from that outlined in Large et al... my plan was
to put the interpolation calls directly in cvmix_coeffs_kpp_low, but I forgot
to do so (and all my testing assumes LMD94 interpolation).