Added API function for changing reaction rates at runtime.

The API function mcell_change_reaction_rate allows to set
reaction rates of *named* reactions at runtime. E.g.
given a named function "foobar", the following code in the
main simulation iteration loop initializes the rate to 3e8 and
then turns it off at a rate of 1e5/timestep.

...

if (world->it_time == 1) {
  if (mcell_change_reaction_rate(world, "foobar", 3e8))a
  {
    mcell_log("+++++ failed to change rate for reaction foobar");
  }
}

if (world->it_time % 100 == 0) {
  if (mcell_change_reaction_rate(world, "foobar", 3e8-(world->it_time*1e5)))
  {
    mcell_log("+++++ failed to change rate for reaction foobar");
  }
}

...

Thus, programmatically at least we can now have arbitrary rate laws.