ENH: 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;