reactingTwoPhaseEulerFoam: Added experimental run-time selectable LTS support

Select LTS via the ddtScheme:

    ddtSchemes
    {
        default         localEuler rDeltaT;
    }

The LTS algorithm is currently controlled with the standard settings in
controlDict, e.g.:

    maxCo           0.5;
    maxDeltaT       2e-8;

with the addition of the optional rDeltaT smoothing coefficient:

    rDeltaTSmoothingCoeff 0.02;

which defaults to 0.02.

    ddtSchemes
    {
        default         localEuler rDeltaT;
    }