Calculate MYSTIC heating rates for each grid box; may need a considerable amount of memory, depending on the 3D cloud grid. Only meaningful with rte_solver montecarlo. mc_heating is identical to mc_absorption for source solar. For source thermal, however, the emission of a photon in a grid box is counted as cooling. An optional argument W_per_m2_and_dz (default), W_per_m3, or K_per_day may be specified which converts the result e.g. to heating rates. This option is only available for forward calculations. For backward please use mc_backward_output heat.