ENH: HashTable and HashSet improvements

- unfriend HashSet, HashTable IO operators

- global min(), max(), minMax() functions taking a labelHashSet and an
  optional limit. For example,

      labelHashSet set = ...;

      Info<< "min is " << min(set) << nl;
      Info<< "max (non-negative) " << max(set, 0) << nl;

- make HashTable iterator '->' dereferencing more consistent by also
  supporting non-pointer types as well.

- read HashTable values in-situ to avoid copying