ENH: added HashTable 'lookup' and 'retain' methods

- lookup(): with a default value (const access)
  For example,
      Map<label> something;
      value = something.lookup(key, -1);

    being equivalent to the following:

      Map<label> something;
      value = -1;  // bad value
      if (something.found(key))
      {
          value = something[key];
      }

    except that lookup also makes it convenient to handle const references.
    Eg,

      const labelList& ids = someHash.lookup(key, labelList());

- For consistency, provide a two parameter HashTable '()' operator.
  The lookup() method is, however, normally preferable when
  const-only access is to be ensured.

- retain(): the counterpart to erase(), it only retains entries
  corresponding to the listed keys.

  For example,
      HashTable<someType> largeCache;
      wordHashSet preserve = ...;

      largeCache.retain(preserve);

    being roughly equivalent to the following two-stage process,
    but with reduced overhead and typing, and fewer potential mistakes.

      HashTable<someType> largeCache;
      wordHashSet preserve = ...;

      {
          wordHashSet cull(largeCache.toc()); // all keys
          cull.erase(preserve);               // except those to preserve
          largeCache.erase(cull);             //
      }

  The HashSet &= operator and retain() are functionally equivalent,
  but retain() also works with dissimilar value types.

applications/test/HashSet/Test-hashSet.C

@@ -197,8 +197,12 @@ int main(int argc, char *argv[])
         Info<< "setD has no 11" << endl;
     }
 
+    Info<< "setB : " << flatOutput(setB) << endl;
     Info<< "setD : " << flatOutput(setD) << endl;
 
+    setD -= setB;
+    Info<< "setD -= setB : " << flatOutput(setD) << endl;
+
     // This should not work (yet?)
     // setD[12] = true;