This is an experimental feature demonstrating the potential of MULES to
create bounded solution which are 2nd-order in time AND space.

Crank-Nicolson may be selected on U and/or alpha but will only be fully
2nd-order if used on both within the PIMPLE-loop to converge the
interaction between the flux and phase-fraction.  Note also that
Crank-Nicolson may not be used with sub-cycling but all the features of
semi-implicit MULES are available in particular MULESCorr and
alphaApplyPrevCorr.

Examples of ddt specification:

ddtSchemes
{
    default         Euler;
}

ddtSchemes
{
    default         CrankNicolson 0.9;
}

ddtSchemes
{
    default         none;
    ddt(alpha)      CrankNicolson 0.9;
    ddt(rho,U)      CrankNicolson 0.9;
}

ddtSchemes
{
    default         none;
    ddt(alpha)      Euler;
    ddt(rho,U)      CrankNicolson 0.9;
}

ddtSchemes
{
    default         none;
    ddt(alpha)      CrankNicolson 0.9;
    ddt(rho,U)      Euler;
}

In these examples a small amount of off-centering in used to stabilize
the Crank-Nicolson scheme.  Also the specification for alpha1 is via the
generic phase-fraction name to ensure in multiphase solvers (when
Crank-Nicolson support is added) the scheme is identical for all phase
fractions.