Calculate MYSTIC absorption of each 3D grid box and write it to file mc_basename.abs.spc
in the following format:
lambda ix iy iz absorption
Only meaningful with rte_solver montecarlo.
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 abs.