Removed extraneous self.string statements in Rate.__init__ and generalized calculation of self.prefactor and self.dens_exp for all chapters. Rate.prefactor is now correct for both the cases ```a + a + b + b --> [...]``` and ```a + a + a + b --> [...]```.
Removed extraneous self.string statements in Rate.__init__ and generalized calculation of self.prefactor and self.dens_exp for all chapters. Rate.prefactor is now correct for both the cases ```a + a + b + b --> [...]``` and ```a + a + a + b --> [...]```.

dwillcox/pyreaclib,pyreaclib/pyreaclib