ENH: improvements in the surface sampling infrastructure

- improvement documentation for surface sampling.

- can now specify alternative sampling scheme for obtaining the
  face values instead of just using the "cell" value. For example,

      sampleScheme    cellPoint;

  This can be useful for cases when the surface is close to a boundary
  cell and there are large gradients in the sampled field.

- distanceSurface now handles non-closed surfaces more robustly.
  Unknown regions (not inside or outside) are marked internally and
  excluded from consideration. This allows use of 'signed' surfaces
  where not previously possible.

tutorials/compressible/rhoSimpleFoam/squareBend/constant/triSurface/angledPlane.obj

@@ -0,0 +1,11 @@
+# An angled plane that extends beyond the geometry
+o angledPlane
+
+v -0.03 -0.08 -0.03
+v -0.03 -0.08 0.03
+v -0.07 -0.02 0.03
+v -0.07 -0.02 -0.03
+
+f 1 2 4
+f 2 3 4
+# EOF

tutorials/compressible/rhoSimpleFoam/squareBend/system/fieldTransfer

@@ -45,7 +45,19 @@ fieldTransfer
             ${_plane}
             bounds (-1 -1 -1) (0 0 1);
         }
+
+        // Angled plane - for general testing
+        plane3
+        {
+            type        distanceSurface;
+            distance    0;
+            signed      true;
+
+            surfaceType triSurfaceMesh;
+            surfaceName angledPlane.obj;
+        }
     );
 }
 
+
 // ************************************************************************* //

tutorials/compressible/rhoSimpleFoam/squareBend/system/sampling

@@ -54,6 +54,19 @@ UI2
 }
 
 
+// Uniformity on sampled surface
+UI3
+{
+    ${__surfaceFieldValue}
+
+    regionType      surface;
+    name            plane3;
+
+    operation       uniformity;
+    fields          ( U T );
+}
+
+
 // Inflow uniformity, but use a scalar field for weighting
 // Since this field is quite uniform, there should be no difference
 T_UI1
@@ -96,4 +109,5 @@ rhoU_UI2
     fields          ( p rho U rhoU );
 }
 
+
 // ************************************************************************* //

tutorials/compressible/rhoSimpleFoam/squareBend/system/samplingDebug

@@ -0,0 +1,51 @@
+// -*- C++ -*-
+
+// ************************************************************************* //
+
+debug
+{
+    type    surfaces;
+    libs    ("libsampling.so");
+    log     true;
+    writeControl  timeStep;
+    writeInterval 1;
+
+    fields  (rho U);
+
+    sampleScheme  cellPoint;
+    interpolationScheme cellPoint;
+    surfaceFormat ensight;
+
+    formatOptions
+    {
+        ensight
+        {
+            collateTimes  true;
+        }
+    }
+
+    surfaces
+    (
+        angledPlane
+        {
+            type        distanceSurface;
+            distance    0;
+            signed      true;
+            regularise  true;
+            surfaceType triSurfaceMesh;
+            surfaceName angledPlane.obj;
+        }
+
+        iso
+        {
+            type        isoSurface;
+            isoField    p;
+            isoValue    1e5;
+            regularise  true;
+            interpolate true;
+        }
+    );
+}
+
+
+// ************************************************************************* //