rhoCentralFoam: Added experimental LTS support

Select LTS via the ddtScheme:

ddtSchemes
{
    default         localEuler rDeltaT;
}

The LTS algorithm is 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.

For cases with reasonably uniform meshes like the forwardStep tutorial
LTS does not provide much benefit but for cases with large variation in
cell-size like the biconic25-55Run35 tutorial LTS provides significant
speed-up to convergence particularly if started from uniform conditions.