Added module rmg.spectral for representing molecular degrees of freedom.
Added module rmg.spectral for representing molecular degrees of freedom.

Currently this module contains the frequency database.

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