Use molar_mass constituents property to replace get_molar_mass_ratio in RRTMGP gas optics

As pointed out by @nusbaume in https://github.com/ESCOMP/atmospheric_physics/pull/230, instead of using the function get_molar_mass_ratio in rrtmgp_lw_gas_optics_data.F90 (which sets the mass ratios per gas), we should set those molar masses in the the constituents registration phase and grab them directly via the constituent properties object's molar_mass property.