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.

applications/utilities/mesh/manipulation/checkMesh/checkTools.C

@@ -250,7 +250,7 @@ void Foam::mergeAndWrite
                 false  // serial - already merged
             );
 
-            writer.writeGeometry();
+            writer.write();
             writer.clear();
         }
     }
@@ -264,7 +264,7 @@ void Foam::mergeAndWrite
             false  // serial - already merged
         );
 
-        writer.writeGeometry();
+        writer.write();
         writer.clear();
     }
 }