semiPermeableBaffle: Added mole-fraction and partial-pressure input options

The semiPermeableBaffleMassFraction boundary condition can now calculate
the mass flux as proportional to the difference in mole fraction or
partial pressure. A mass fraction difference driven transfer is also
still possible. An additional keyword, "input" has been added which is
used to select the variable used to calculate the transfer. An example
specification is as follows:

    baffle
    {
        type            semiPermeableBaffleMassFraction;
        samplePatch     membranePipe;
        c               0.1;
        input           massFraction;
        value           uniform 0;
    }

In order to facilitate this, a "W" method to get the molar mass on a
patch has been added to the thermodynamics. To avoid name-clashes,
methods that generate per-species molar masses have been renamed "Wi".

This work was supported by Georg Skillas, at Evonik

tutorials/combustion/reactingFoam/RAS/membrane/0/CH4.orig

@@ -54,6 +54,7 @@ boundaryField
         type            semiPermeableBaffleMassFraction;
         samplePatch     membranePipe;
         c               0.1;
+        input           massFraction;
         value           uniform $:sleeve.CH4;
     }
     membranePipe
@@ -61,6 +62,7 @@ boundaryField
         type            semiPermeableBaffleMassFraction;
         samplePatch     membraneSleeve;
         c               0.1;
+        input           massFraction;
         value           uniform $:pipe.CH4;
     }
 }