Fixed bug in evaluation of species fluxes in solver.
Fixed bug in evaluation of species fluxes in solver.

You don't want to divide by the characteristic rate since it might be zero
(i.e. if the core has no reactions). Even if the characteristic rate is
nonzero, we still don't want to divide by it because we already compare to
it at a later step.

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