create_cohort() and the setting of canopy layer

[rgknox]

We pass in the canopy layer when we call create_cohort.

https://github.com/NGEET/fates/blob/sci.1.67.2_api.27.0.0/biogeochem/EDCohortDynamicsMod.F90#L160-L162

During cold start initializations, we pass a value of 1 (canopy). During recruitment, we pass in the value of patch%NCL_p which is the lowest canopy layer of the patch. Either way, if we are creating a new cohort, it seems its canopy layer is going to be an emergent property that shouldn't really be set until we run the canopy layering scheme anyway. I wonder if we can remove the argument and just set the canopy layer to uninitialized (ie a negative value).