Create getSymmetryNumber() function for species object.
Create getSymmetryNumber() function for species object.

Returns the largest symmetry number amongst all resonance isomers.  This will return
the aromatic species' symmetry number if that is the largest one.

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