ENH: allow wordHashSet filter for IOobjectList::names

- simplifies usage.
  Support syncPar check on names() to detect inconsistencies.

- simplify readFields, ReadFields and other routines by using these
  new methods.

applications/utilities/postProcessing/dataConversion/foamToEnsight/foamToEnsight.C

@@ -316,7 +316,7 @@ int main(int argc, char *argv[])
         // and doesn't cost much) and simultaneously remove all
         // "*_0" restart fields
 
-        for (auto fieldType : volFieldTypes)
+        for (const word& fieldType : volFieldTypes)
         {
             usableObjects
             (
@@ -360,7 +360,7 @@ int main(int argc, char *argv[])
         // ~~~~~~~~~~~~~~~~~~~~~~
         Info<< "Write volume field (";
 
-        for (auto fieldType : volFieldTypes)
+        for (const word& fieldType : volFieldTypes)
         {
             // For convenience, just force each field-type into existence.
             // This simplifies code logic and doesn't cost much at all.