2018-05-02, Patrick:
The handling of extinction and transmission inside the MC methods should be
updated and use the new features by Richard. That is, to use propmat and e.g. the
function ext2trans should be removed.

2018-05-02, Patrick:
Revise how MAINTAG, SUBTAG ... inside jacobian_quantities are used. The options
for MAINTAG should be something like: atmosphere, spectroscopy, surface, sensor.