CrankNicolsonDdtScheme: Added option "ramp" function to smoothly transition from Euler

    Off-centering is specified via the mandatory coefficient \c ocCoeff in the
    range [0,1] following the scheme name e.g.
    \verbatim
    ddtSchemes
    {
        default         CrankNicolson 0.9;
    }
    \endverbatim
    or with an optional "ramp" function to transition from the Euler scheme to
    Crank-Nicolson over a initial period to avoid start-up problems, e.g.
    \verbatim
    ddtSchemes
    {
        default         CrankNicolson
        ocCoeff
        {
            type scale;
            scale linearRamp;
            duration 0.01;
            value 0.9;
        };
    }
    \endverbatim

Note this functionality is experimental and the specification and implementation
may change if issues arise.