do not calculate the atom count when constraint is not set
do not calculate the atom count when constraint is not set

previously, the atom count for C, H, N, ... was calculated for every
molecule, regardless of whether there was a constraint in place.

The default value for a constraint is set to -1, so that it can be used
to determine whether an atom count should be calculated or not.

pierrelb/RMG-Py,nyee/RMG-Py,nickvandewiele/RMG-Py,nickvandewiele/RMG-Py,nyee/RMG-Py,pierrelb/RMG-Py