Introduced `getNumberDegreesOfFreedom` into conformer class

This partially addresses issue #213.

This function gives the total molecular degrees of freedom for a
species. I think it belongs in the conformer class, but please let me
know if you think otherwise. I've also included a couple unittests for
ethane and ethylene. I've recompiled RMG-Py and ensured that it's
working. That it's working is one thing, but apologies in advance for
the poor programming style :)

The next thing I plan to do is to use this function to try and close #213