Added toArrhenius(Tmin,Tmax) method to MultiArrhenius kinetics.

This fits a single Arrhenius expression to the kinetics data from the
MultiArrhenius kinetics.

Wasn't sure of the best way to get the units of k from a kinetics object.
Seems like this could be useful elsewhere, and perhaps should be in 
the base parent class.
Convert kinetics to Arrhenius and check positive barrier inside PDep code.

Rather than trying to do this when loading a reaction library (which causes problems
when the same reaction is in a seed mechanism) we do the conversion and check
at the last minute, just before running the PDep calculation.
Report when a library reaction is skipped because it's already in the model.

These are usually reactions that are in a seed mechanism,
but could be in a previous reaction library. In either case,
they won't be added to the edge.
Don't check barrier heights when adding reaction library to edge.

We now check for negative barriers inside the PDep code.
That means that this bit of Library code doesn't have any "is pressureDependence on"
logic, which feels better.