Important change to Fluids
- The user can now send a propname argument to add_method
- The will create a new property attribute on the fluid with the propname that stores the model requested in add_method
- This will also force the model to write it's values to the propname dictionary
- This change is to allow calculation of same properties for different species in the same fluid (ie. diffusivity becomes diff_A and diff_B, for instance).  Both can use the same model (ie fuller) or different ones.  They are independent.