ENH: snappyHexMesh: optionally remove 'small' regions. Fixes #1772.

This is for a very specific use case where the faceZones are
imprinted after meshing the normal geometry. This sometimes
splits off badly connected bits of the mesh. One way to remove
these is to use e.g. subsetMesh. This embeds the
same functionality inside snappyHexMesh.

src/mesh/snappyHexMesh/snappyHexMeshDriver/refinementParameters/refinementParameters.C

@@ -94,6 +94,7 @@ Foam::refinementParameters::refinementParameters
     ),
     nErodeCellZone_(dict.getOrDefault<label>("nCellZoneErodeIter", 0)),
     nFilterIter_(dict.getOrDefault<label>("nFilterIter", 2)),
+    minCellFraction_(dict.getOrDefault<scalar>("minCellFraction", 0)),
     dryRun_(dryRun)
 {
     point locationInMesh;