BUG: incorrect HashTable / HashSet instances

- Eg instead of using labelHashSet, used HashSet<label> which uses
  the string::hash for hashing. Other places inadvertently using the
  string::hash instead of Hash<label> for hashing.

STYLE: use Map<..> instead of HashTable<.., label, Hash<label>>

- reduces clutter

src/functionObjects/field/particleDistribution/particleDistribution.C

@@ -146,7 +146,7 @@ bool Foam::functionObjects::particleDistribution::write()
         // Tag field present - generate distribution per tag
         const IOField<label>& tag =
             cloudObr.lookupObject<IOField<label>>(tagFieldName_);
-        const HashSet<label> tagMap(tag);
+        const labelHashSet tagMap(tag);
         const label tagMax = tagMap.size();
 
         List<DynamicList<label>> tagAddr(tagMax);