Added first versions of classes for various molecular degress of freedom.
Added first versions of classes for various molecular degress of freedom.

The degrees of freedom currently implemented are:

* HarmonicOscillator - 1D quantum harmonic oscillator
* HinderedRotor - 1D classical Pitzer rotor
* Translation - any dimensionality
* RigidRotor - various forms

No guarantees that these classes are fully and correctly implemented yet. In
particular, RigidRotor is probably wrong.

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