Test C balance closure

[dlebauer]

Add an internal diagnostic to check C mass balance each timestep.

Check that the following is true:

C_t - C_(t-1) = inputs - outputs + tolerance

I think tolerance could be on the order of approximately max(GPP)/1000. Smaller is okay.

C_total sums all modeled C pools and inputs/outputs include photosynthesis, respiration, harvest.

Done when internal check passes after #208 is fixed (or otherwise resolved).