ENH: semiPermeableBaffle: Added two new boundary conditions and a tutorial

Two boundary conditions for the modelling of semi-permeable baffles have
been added. These baffles are permeable to a number of species within
the flow, and are impermeable to others. The flux of a given species is
calculated as a constant multipled by the drop in mass fraction across
the baffle.

The species mass-fraction condition requires the transfer constant and
the name of the patch on the other side of the baffle:

boundaryField
{
    // ...

    membraneA
    {
        type            semiPermeableBaffleMassFraction;
        samplePatch     membranePipe;
        c               0.1;
        value           uniform 0;
    }
    membraneB
    {
        type            semiPermeableBaffleMassFraction;
        samplePatch     membraneSleeve;
        c               0.1;
        value           uniform 1;
    }
}

If the value of c is omitted, or set to zero, then the patch is
considered impermeable to the species in question. The samplePatch entry
can also be omitted in this case.

The velocity condition does not require any special input:

boundaryField
{
    // ...

    membraneA
    {
        type            semiPermeableBaffleVelocity;
        value           uniform (0 0 0);
    }
    membraneB
    {
        type            semiPermeableBaffleVelocity;
        value           uniform (0 0 0);
    }
}

These two boundary conditions must be used in conjunction, and the
mass-fraction condition must be applied to all species in the
simulation. The calculation will fail with an error message if either is
used in isolation.

A tutorial, combustion/reactingFoam/RAS/membrane, has been added which
demonstrates this transfer process.

This work was done with support from Stefan Lipp, at BASF.