ENH: support VTK output of point ids

- can be useful for diagnosing mesh internals and the locations of
  decomposed cells.

applications/utilities/postProcessing/dataConversion/foamToVTK/convertVolumeFields.H

@@ -313,7 +313,13 @@ Description
             internalWriter->beginPointData
             (
                 nVolFields + nDimFields + nPointFields
+              + (withPointIds ? 1 : 0)
             );
+
+            if (withPointIds)
+            {
+                internalWriter->writePointIDs();
+            }
         }
 
         forAll(patchWriters, writeri)

applications/utilities/postProcessing/dataConversion/foamToVTK/foamToVTK.C

@@ -91,6 +91,9 @@ Usage
       - \par -no-point-data
         Suppress conversion of pointFields. No interpolated PointData.
 
+      - \par -with-point-ids
+        Additional pointID field for internal mesh
+
       - \par -poly-decomp
         Decompose polyhedral cells into tets/pyramids
 
@@ -379,6 +382,12 @@ int main(int argc, char *argv[])
     );
     argList::addOptionCompat("no-point-data", {"noPointValues", 1806});
 
+    argList::addBoolOption
+    (
+        "with-point-ids",
+        "Additional pointID field for internal mesh"
+    );
+
     argList::addBoolOption
     (
         "one-boundary",  // allPatches
@@ -495,6 +504,19 @@ int main(int argc, char *argv[])
             << nl;
     }
 
+    const bool withPointIds = args.found("with-point-ids");
+    if (withPointIds)
+    {
+        Info<< "Write point ids requested";
+
+        if (noPointValues)
+        {
+            Info<< ", but ignored due to the '-no-point-data' option";
+        }
+
+        Info<< nl;
+    }
+
     wordRes includePatches, excludePatches;
     if (doBoundary)
     {