optimized compute_reaction_rate
optimized compute_reaction_rate

turned out to be a CPU bottleneck

- removed assertion
- removed expensive np.absolute (for arrays)
- removed need for concentrations array
- moved calc of k to after the conc calc
- removed isReactant variable

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