ENH: expose IntRange {begin,end}_value() methods

- end_value() corresponds to the infrequently used after() method, but
  with naming that corresponds better to iterator naming conventions.

  Eg,

     List<Type> list = ...;
     labelRange range = ...;

     std::transform
     (
         (list.data() + range.begin_value()),
         (list.data() + range.end_value()),

         outIter,
         op
     );

- promote min()/max() methods from labelRange to IntRange base class

STYLE: change timeSelector from "is-a" to "has-a" scalarRanges.