ENH: improved consistency of surface writers (#1232)

- remove writeGeometry() in favour of write() and make it pure virtual
  so that all writers must explicitly deal with it.

- establish proxy extension at construction time and treated as an
  invariant thereafter. This avoids potentially surprising changes in
  behaviour when writing.

src/sampling/sampledSurface/sampledSurfaces/sampledSurfaces.C

@@ -560,7 +560,7 @@ bool Foam::sampledSurfaces::performAction(unsigned request)
             // Write geometry if no fields would otherwise be written
             if (!outWriter.nFields() || outWriter.separateGeometry())
             {
-                outWriter.writeGeometry();
+                outWriter.write();
                 continue;
             }