ENH: provide setter methods for IOobject read/write options etc.

- simplifies local toggling.

- centralize fileModification static variables into IOobject.
  They were previously scattered between IOobject and regIOobject

applications/utilities/postProcessing/dataConversion/foamToGMV/moveMesh.H

@@ -14,7 +14,7 @@
     {
         Info<< "new points available" << endl;
         // Read new points
-        io.readOpt() = IOobject::MUST_READ;
+        io.readOpt(IOobject::MUST_READ);
         mesh.movePoints(pointIOField(io));
     }
 }