ENH: improvements for labelRange

- constexpr, noexcept.
  Added an 'at()' method for returning an iterator within the range
  and changed operator()(label) to have behaviour as per found().
  This makes the labelRange usable as a unary predicate.

- added templated conversion class 'toLabelRange'

- add range() method to polyPatch and surfZone classes, and corresponding
  templated conversion functors.
  For example,

      auto patchDims = ListOps::create<labelRange>
      (
          mesh.boundaryMesh(),
          toLabelRange<polyPatch>()
      );

  to create a List<labelRange> representing the patch extents.