Reaction.generatePairs() can now handle more than two reactants/products.
Reaction.generatePairs() can now handle more than two reactants/products.

Occasionally you will find a reaction of the form A + B -> C + D + E in
a reaction library. We need to be able to at least hazard a guess as to
the flux pairs for this. The new algorithm will pair off reactants and
products by heavy atom count until only a single reactant or product
remains, and then pair the remaining products/reactants off with that
reactant/product. Probably not correct in a bunch of cases, but better
than nothing.

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