Implement support for setting constant concentrations for PDModels

[DavAug]

• PMC gets another optional argument, which is Drug concentration
• This only matters for PD models
• Similar to Dose amount and Duration for PK models, Drug concentration is set just before the models are composed.
• It requires that the pk_output variable is specfied.
• When Drug concentration is provided, the pk_output variable is fixed to that value.