ENH: new cuttingPlane cutting scheme

- takes a direct approach of determining which cells are cut and walks
  the cell faces directly to build the resulting surface.

- better handling of corner cases.
  * Avoids redundant points when the cut passes exactly through a
    mesh point.
  * Supresses generation of duplicates faces when the plane cut
    coincides exactly with a mesh face.

- for severely concave cells where the plane cuts a face multiple times
  there is currently no remedial action taken, except to note the
  failure and unwind the insertion of the corresponding points and
  faces.

tutorials/compressible/rhoSimpleFoam/squareBend/system/samplingDebug

@@ -26,6 +26,23 @@ debug
         }
     }
 
+    _plane
+    {
+        type    plane;
+        source  cells;
+        triangulate false;
+
+        planeType   pointAndNormal;
+
+        pointAndNormalDict
+        {
+            normal (-1 0 0);
+            point  (-0.042 0 0);
+            // point  (-0.0425 0 0);  // between faces
+        }
+    }
+
+
     surfaces
     (
         angledPlane
@@ -46,6 +63,20 @@ debug
             regularise  true;
             interpolate true;
         }
+
+        // Top channel
+        plane1
+        {
+            ${_plane}
+            bounds (-1 0 -1) (0 1 1);
+        }
+
+        // Bottom channel
+        plane2
+        {
+            ${_plane}
+            bounds (-1 -1 -1) (0 0 1);
+        }
     );
 }