[index][equals] Add the support for Segments as Indexables in the rtree.

This commit also adds the support for equals(Segment, Segment).

During the creation of the rtree all metrics are calculated for Segments viewed as bounding Boxes.
During the querying Segments are passed into the BG algorithms.

This commit also fixes a compilation error related to the calculation of the Box/Box intersection in the index::detail::intersection_content().
Add indexed_point_view<> which allows to see Boxes and Segments as one of the Points.
Move normalized_view<> to views/detail.
Add index::detail::bounded_view<> which allows to see simple Geometry e.g. a Segment as its bounding Geometry e.g. a Box.

TODO/RECONSIDER:
1. This commit adds return_ref_or_bounds() which returns const& for Points and Boxes and bounding Box view for Segment. The resulting object is then passed into various algorithms like covered_by() or within(). Whenever possible the algorithms should be implemented for the input parameters and the call to return_ref_or_bounds() removed.
2. It's possible that for Segments metrics (like the amount of overlap in the leaf node) could be calculated differently than for Boxes and naiive conversion to bounding Boxes is not fully correct.
3. The way how bounded_view<> works may be not correct for non-cartesian CS. Now, for Segments it just picks MIN and MAX coordinates on the fly which may not be ok e.g. for Segments defined in the spherical CS because of the "winding" of the coordinates.
Merge branch 'develop' of github.com:boostorg/geometry into develop
[equals][index/examples] Optimize equals(Seg,Seg) and fix index/examples

Use less comparisions in equals(Seg,Seg) - suggested by Menelaos Karavelas
Add missing include to examples - caused by the recent change of the distance/rtree
[index] Update description and parameters names of nearest() predicate
[index] Remove unneeded #includes from the rtree.hpp