ENH: refactor cell selections into cellBitSet

- ensightWrite, vtkWrite, fv::cellSetOption

ENH: additional topoSet "ignore" action

- this no-op can be used to skip an action step, instead of removing
  the entire entry

tutorials/compressible/rhoPimpleFoam/RAS/mixerVessel2D/system/scalarTransport

@@ -26,6 +26,7 @@ tracer0
             {
                 cylinder
                 {
+                    action  use;
                     source  cylinder;
                     point1  (0.050 0 -1);
                     point2  (0.050 0  1);