caseDicts/postProcessing: Added triSurfaceAverage packaged function object

This is a packaged function object that conveniently computes averages
of fields on tri-surfaces. It can be executed on the command line as
follows:

    foamPostProcess -func "triSurfaceAverage(name=mid.obj, p)"

This will compute the average of the field "p" on a surface file in
"constant/geometry/mid.obj".

The calculation could also be done at run-time by adding the following
entry to the functions section of the system/controlDict

    #includeFunc triSurfaceAverage(name=mid.obj, p)

etc/caseDicts/postProcessing/surfaceFieldValue/triSurfaceAverage

@@ -0,0 +1,23 @@
+/*--------------------------------*- C++ -*----------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     | Website:  https://openfoam.org
+    \\  /    A nd           | Version:  dev
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+Description
+    Calculates the average on a specified triangulated surface by interpolating
+    onto the triangles and integrating over the surface area. Triangles need to
+    be small (<= cell size) for an accurate result.
+
+\*---------------------------------------------------------------------------*/
+
+name    <triSurfaceFileName>;
+
+fields  (<fieldNames>);
+
+operation areaAverage;
+
+#includeEtc "caseDicts/postProcessing/surfaceFieldValue/triSurfaceValue.cfg"
+
+// ************************************************************************* //

test/postProcessing/channel/system/controlDict

@@ -398,6 +398,8 @@ surfaceFieldValueFunctions
 
     #includeFunc faceZoneFlowRate(name=f0)
 
+    #includeFunc triSurfaceAverage(name=mid.obj, p)
+
     #includeFunc triSurfaceVolumetricFlowRate(name=mid.obj)
 
     #includeFunc triSurfaceDifference