Point iterator (change of file/dir layout and implementation design)

In this PR:
* I update the file/directory layout and implementation design for the point iterator (the `boost::geometry::detail::point_iterator` namespace is introduced and used).
* The methods and contructors for the `point_iterator<>` and `point_reverse_iterator<>` classes are now declared as inline.
* The `segment_iterator<>` implementation is updated following the changes above.