Added Network class to rmg.unirxn.network.
Added Network class to rmg.unirxn.network.

This class provides a representation of a unimolecular reaction network, and
methods to work with it. The currently implemented methods fall into two
categories: manipulating the energy grains and calculating the net rate
coefficients.

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