chemistryModel: Added new option to specify the initial ODE integration time-step

In constant/chemistryProperties in addition to the specification of the initial
ODE integration time-step used at the start of the run:

    initialChemicalTimeStep 1e-12;

this time step may now also be specified for every chemistry integration by
setting the optional entry maxChemicalTimeStep, e.g.

    maxChemicalTimeStep 1e-12;

etc/controlDict

@@ -788,6 +788,7 @@ DebugSwitches
     solidBodyMotionFunction 0;
     solidBodyMotionFvMesh 0;
     solution            0;
+    specie              0;
     spectEddyVisc       0;
     sphereToCell        0;
     spherical           0;