BUG: ensightWrite not reading "excludeFields" (fixes #2696)

- field blocking/exclusion added in commit d9ab5d54ef,
  but was incorrectly doing a lookup for "blockField" for ensight
  although "excludeFields" was documented (and expected).

  Now corrected to use "excludeFields"

src/functionObjects/utilities/ensightWrite/ensightWriteUpdate.C

@@ -103,7 +103,7 @@ bool Foam::functionObjects::ensightWrite::readSelection(const dictionary& dict)
     selectFields_.uniq();
 
     blockFields_.clear();
-    dict.readIfPresent("blockFields", blockFields_);
+    dict.readIfPresent("excludeFields", blockFields_);
     blockFields_.uniq();
 
     // Actions to define selection