Refine Plant Functional Type representation in the T Model submodule.

[davidorme]

A better plant functional type (PFT) representation. The TModelTraits constants class does this at the moment, and may not need much changing. It is unusual though - all the other constants classes only have a single instance for any one analysis, but the demographic model will need something like a Flora object that provides a dictionary onto the complete set of PFT instances to be used in calculation.