- replace internal labelPair typedef (std::pair<label,label>, not
  Foam::labelPair) with range_type to avoid possible confusion

- use internal typedefs for OffsetStrategy and DataComponent::base_ptr
  to avoid additional clutter in Foam namespace.

- pass ownership of std::unique_ptr by move

STYLE: use class instead of struct with private/protected content

ENH: fix naive partitioning calculations (had some rounding issues)