Change moss C initialization.
Change moss C initialization.

Changes soil carbon initialization logic. Previously, we were assigning
values to the soil layer's C pools (raw, soma, somcr, sompr) based on
layer thickness (as for other organic layers). With this commit, we
change to a match our current understanding of the intended structure:
 - There are two moss layers: living (upper) and dead (lower)
 - Moss layers do NOT have the raw, soma, somcr and sompr C pools that
   other soil layers have.
 - The C pool in the moss layer is "dmossc" (dead moss carbon), and is
   set via the parameter variable: initdmossc (cmt_bgcsoil.txt)
 - the thickness of the moss layers comes from parameter

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