Fixed units bug in k(T,P) equilibrium check error message.

Pressures are stored internally in Pa, not bar.
Sort reactants and products when loading reactions from CanTherm input.

Otherwise some bimolecular reactant channel comparisons could randomly
fail.
Fixed bug when saving PDepArrhenius kinetics in CanTherm.

The n parameter in an Arrhenius expression is a ScalarQuantity, so you
must use its value_si attribute to access its float value.
Added KineticsGroups.generateGroupAdditivityValues() method.

This is being moved from the kineticsGroups.py script in RMG-database to
the KineticsGroups class in RMG-Py. The code has also been updated to
actually work (several things were out of date).
Added KineticsFamily.getRateCoefficientUnits() method.

This provides a way to determine the typical units of a rate coefficient
for that family: s^-1 if unimolecular or m^3/(mol*s) if bimolecular.
The current implementation uses hardcoded lists that are available in
rmgpy.data.kinetics.common; perhaps a future implementation will find a
better way, but at least you can now call a method and not care about the
implementation details most of the time.
Restored KineticsDatabase.getForwardReactionForFamilyEntry() to working order.

This function is used heavily in the kinetics group additivity generation
and evaluation code. (It also could use some refactoring, but that
comes after things are working again.)