ENH: cellLimitedGrad gradientLimiters: Added support for run-time selectable...

ENH: cellLimitedGrad gradientLimiters: Added support for run-time selectable gradient limiter function

Minmod is the default limiter function and specified with an explicit name e.g.:

    gradSchemes
    {
        default Gauss linear;
        limited cellLimited Gauss linear 1;
    }

Venkatakrishnan and cubic limiter functions are also provided and may be
specified explicitly e.g.:

    gradSchemes
    {
        default Gauss linear;
        limited cellLimited<Venkatakrishnan> Gauss linear 1;
    }

or

    gradSchemes
    {
        default Gauss linear;
        limited cellLimited<cubic> 1.5 Gauss linear 1;
    }

The standard minmod function is recommended for most applications but if
convergence or stability problems arise it may be beneficial to use one of the
alternatives which smooth the gradient limiting.  The Venkatakrishnan is not
well formulated and allows the limiter to exceed 1 whereas the cubic limiter is
designed to obey all the value and gradient constraints on the limiter function,
see

    Michalak, K., & Ollivier-Gooch, C. (2008).
    Limiters for unstructured higher-order accurate solutions
    of the Euler equations.
    In 46th AIAA Aerospace Sciences Meeting and Exhibit (p. 776).

The cubic limiter function requires the transition point at which the limiter
function reaches 1 is an input parameter which should be set to a value between
1 and 2 although values larger than 2 are physical but likely to significantly
reduce the accuracy of the scheme.

VenkatakrishnanGradientLimiter: Updated documentation

cubicGradientLimiter: Documented private data