etc/caseDicts/postProcessing/fields: New 'add' and 'subtract' functionObjects

etc/caseDicts/postProcessing/fields/add

+/*--------------------------------*- C++ -*----------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Web:      www.OpenFOAM.org
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+Description
+    Add a list of fields.
+
+\*---------------------------------------------------------------------------*/
+
+type            add;
+libs            ("libfieldFunctionObjects.so");
+
+fields          (<field names>);
+
+executeControl  writeTime;
+writeControl    writeTime;
+
+// ************************************************************************* //

etc/caseDicts/postProcessing/fields/subtract

+/*--------------------------------*- C++ -*----------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | Web:      www.OpenFOAM.org
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+Description
+    From the first field subtract the remaining fields in the list.
+
+\*---------------------------------------------------------------------------*/
+
+type            subtract;
+libs            ("libfieldFunctionObjects.so");
+
+fields          (<field names>);
+
+executeControl  writeTime;
+writeControl    writeTime;
+
+// ************************************************************************* //