Added and refactored lots of unit tests for the rmgpy.kinetics module.
Added and refactored lots of unit tests for the rmgpy.kinetics module.

With this commit every method of each kinetics model class has a
corresponding unit test, and all of them pass successfully. Along the
way these unit tests helped me find and address a few of the bugs fixed
in the last commit, including a few that would have been less than
obvious to spot in other settings. The value of test-driven development!

pierrelb/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,comocheng/RMG-Py,enochd/RMG-Py,enochd/RMG-Py,comocheng/RMG-Py,nickvandewiele/RMG-Py,chatelak/RMG-Py,faribas/RMG-Py,pierrelb/RMG-Py,chatelak/RMG-Py,KEHANG/RMG-Py,nyee/RMG-Py,faribas/RMG-Py,KEHANG/RMG-Py