ENH: added classes() method to objectRegistry/IOobjectList

- provides a summary hash of classes used and their associated object names.

  The HashTable representation allows us to leverage various HashTable
  methods. This hashed summary view can be useful when querying
  particular aspects, but is most useful when reducing the objects in
  consideration to a particular subset. For example,

      const wordHashSet interestingTypes
      {
          volScalarField::typeName,
          volVectorField::typeName
      };

      IOobjectList objects(runTime, runTime.timeName());
      HashTable<wordHashSet> classes = objects.classes();

      classes.retain(interestingTypes);

      // Or do just the opposite:
      classes.erase(unsupportedTypes);

  Can also use the underlying HashTable filter methods

STYLE: use templated internals to avoid findString() when matching subsets