[strategies][distance][spherical][cross_track] prototype implementation
for the new cross_track strategy; the new strategy defines a comparable
counterpart which is then used to compute the cross track distance;
the old code is kept for now (guarded by the macro
BOOST_GEOMETRY_USE_OLD_CROSS_TRACK_STRATEGY);
[strategies][distance][spherical][cross_track] add a missing space
[strategies][distance][spherical][cross_track] remove duplicate word in comments section
Merge branch 'develop' of https://github.com/boostorg/geometry into feature/new_cross_track_strategy
[test][algorithms][distance][spherical equatorial] print, in debug mode, the
radius of the strategy; change the label in one of the test cases;
[strategies][distance][spherical][cross track] update comments' section and make it
a stand-alone explanatory document; polish code; remove old code; fix header includes;
fix long lines;
[strategies][distance][spherical][cross track] fix typo
[test][algorithms][distance][spherical equatorial] update unit test to account
for the changes in the cross track strategy (that now has a comparable counterpart)
[strategies][distance][spherical][cross track] fix typo
[io] Fix reference to reference issue for older MSVC.
[test][index] Replace struct with class keyword in specialization of allocators<> for throwing node.
[buffer] refactor analysis of piece into shorter and slightly more efficient (few %) code.
It is now one function. In later commits we will extend this by checking helper
segments separately, which should also be more efficient.

It changes one test-case, probably because ofn ear_offsetted. The change is not visible
and the result is probably more correct. SQL Server reports this polygon as invalid.
Merge branch 'develop' of github.com:boostorg/geometry into develop
[buffer] Piece visitor, remove unused types and we still need to check on geometry_code
[buffer] extend piece analyses, check helper-segments for on-offsetted, on-original,
disjointness. This removes the workaround w.r.t. has_concave
[buffer][fix] oops, use geometry instead of bg
[buffer] add specific robust bounding box for offsetted part.
This gains in some cases more than 10% performance

This gives two small changes in expected areas for country buffer deflate-5, which are not
visible and not detectable using other tools (spent quite some time to try)
[buffer] add cheap check using the new robust_offsetted_envelope
This safes about 1% performance
It is only applicable for convex joins, we will add this distinction later
(currently all joins are convex)
[buffer] Skip pieces not used for turn-in-piece in a more early phase in
partition. This can in some cases enhance performance more than 40%
[buffer] remove now unused neighbour variable and remove now non-used code,
now that analysis is done more early