ENH: Avoid constant/polyMesh/blockMeshDict (issue #309)

- relocate to system/blockMeshDict, which avoids it being cleaned out
  accidentally

tutorials/mesh/parallel/cavity/Allclean

@@ -1,12 +1,13 @@
 #!/bin/sh
+cd ${0%/*} || exit 1    # run from this directory
 
 # Source tutorial clean functions
 . $WM_PROJECT_DIR/bin/tools/CleanFunctions
 
+cleanCase
+
 # Restore default dictionaries
 cp system/decomposeParDict-2 system/decomposeParDict
 cp system/controlDict-startTime system/controlDict
 
-cleanCase
-rm -f constant/polyMesh/boundary
-rm -f constant/polyMesh/procAddressing
+# -----------------------------------------------------------------------------