ENH: rationalize cloud method inheritance

- a top-level cloud::nParcels() virtual, which is overloaded by the
  first level of Cloud inheritance. This permits quick determination of
  cloud sizes, even when retrieved from registry with the base level.
  Eg,
      cloud* cldPtr = mesh.cfindObject<cloud>("myCloud");
      label nParcels = (cldPtr ? cldPtr->nParcels() : 0);

- make writeLagrangianPositions on by default unless explicitly
  disabled in the InfoSwitches.

  Flag output errors (where neither coordinates nor positions are
  written) with Fatal.

- additional IOField helper functions in cloud

STYLE: simplify iterator inheritance

src/functionObjects/graphics/runTimePostProcessing/geometryCloud.C

@@ -159,7 +159,7 @@ addGeometry
 
     objPtr->writeObjects(obrTmp);
 
-    const auto* pointsPtr = obrTmp.findObject<vectorField>("position");
+    const auto* pointsPtr = cloud::findIOPosition(obrTmp);
 
     if (!pointsPtr)
     {

src/functionObjects/graphics/runTimePostProcessing/geometryCloudGather.C

@@ -46,7 +46,7 @@ Foam::functionObjects::runTimePostPro::geometryCloud::gatherCloud
     auto multiPiece = vtkSmartPointer<vtkMultiPieceDataSet>::New();
     multiPiece->SetNumberOfPieces(Pstream::nProcs());
 
-    const auto* pointsPtr = obrTmp.findObject<vectorField>("position");
+    const auto* pointsPtr = cloud::findIOPosition(obrTmp);
 
     if (!needsCollective())
     {