Add new land realization (transition matrix) and rewrite of the som module

[k4rst3ns]

• include new land realization featuring land transition matrix
• include rewrite of som module featuring update of carbon stocks via som module
• smaller changes in other modules to remove soil carbon related calculations there and bundle them in som
• new settings for ghg_policy (including bugfix of old ssp default setting, which overestimates soil emissions)

[flohump]

The changes in the "land" and the "som" module seem fine to me.
I only have some minor comments.