Changeset 2758 for perso

several bugfixes to ensure biomass and circ_class_biomass are consistent;
however, still minor deviations between both variables can occur.
These deviations are caused by the not-optimal way of calculating the changes
in these biomass pools seperately rather than diagnosing the changes from
each other. This problem is in many subroutines thus I won't fix it althoug this deviation
lead to violations of mass conservation.

DSG