reactingFoam::setRDeltaT: Add support for limiting the local time-step by the reaction rates

e.g. in the reactingFoam/laminar/counterFlowFlame2DLTS tutorial:

PIMPLE
{
    momentumPredictor no;
    nOuterCorrectors  1;
    nCorrectors     1;
    nNonOrthogonalCorrectors 0;

    maxDeltaT       1e-2;
    maxCo           1;
    alphaTemp       0.05;
    alphaY          0.05;
    Yref
    {
        O2          0.1;
        ".*"        1;
    }
    rDeltaTSmoothingCoeff 1;
    rDeltaTDampingCoeff 1;
}

will limit the LTS time-step according to the rate of consumption of 'O2'
normalized by the reference mass-fraction of 0.1 and all other species
normalized by the reference mass-fraction of 1.  Additionally the time-step
factor of 'alphaY' is applied to all species.  Only the species specified in the
'Yref' sub-dictionary are included in the LTS limiter and if 'alphaY' is omitted
or set to 1 the reaction rates are not included in the LTS limiter.