[algorithms][is_valid] use bg::partition to check whether the interior
rings of a polygon (that are disjoint from each other and from the
exterior ring) do not overlap with each other
[algorithms][is_valid] fix bug in collecting ring iterators not
associated with turns; rename overlaps_ring to overlaps_box;
rewrite expand_box and overlaps_box in a more generic way (the
arguments are considered now as generic iterators, not necessarily
iterators or rings);
[algorithms][is_valid] make the visitor for partition more generic
(semantically) so that it fits the semantics of the visitor needed
for partition in the multipolygon case
[algorithms][is_valid] use bg::partition to test whether polygons
(not associated with any turns) do not intersect/overlap each other
Merge branch 'develop' of https://github.com/boostorg/geometry into feature/is_valid
[buffer][test] change usage of join strategies for robustness check
[buffer] remove unused variable
[buffer] don't check neighbor for on-border
[buffer][test] remove unused variable
[buffer][fix] In rare cases an extra ring was traversed and created. Fixed, that ring is
discarded now. This fixes behaviour of assign_parents with positive-rings in positive-rings (when
check_for_orientation is true; normally (for overlay) it is false)

It also reverses a negative-only ring so fixes a linestring_buffer testcase.

For multi-polygon it fixes new testcase rt_u2 found by recursive-polygon-buffer test
[buffer][test] clean up old comments/cases
[buffer][fix] Fix accidental removal of == 0
[buffer][fix] Fix turns which were only on helper-segments, they can be discarded.
This fixes rt_u3 and rt_u4 where a false interior ring was generated
[buffer][fix] Distinguish between "within" and "within_near_offsetted"
The last one might be offsetted by 1 due to integer rounding. It is not
selectable as a starting point but may be used for traversal
[buffer][fix] use taxicab distance instead of comparable distance
This is faster and avoids overflows. This fixes rt_u6
[buffer][test] improve piece labelling on round joins
[buffer][test] Minor change in comments/whitespace
[buffer][test] Add rt_u7 which is currently failing because of tripled IP,
needing get_left_turn info again. Added debug info
[test][robustness] move overlay/robustness folder to robustness folder
[test][robustness] updated tolerance