[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
[test][strategies][spherical][cross track] add test cases where the distance
of the point from the spherical segment is small (1e-3 or smaller)
[strategies][distance][spherical][cross track] modify computation of point-to-segment
distance to be numerically robust (this applies to the case where the point projects
inside the segment)
Merge pull request #182 from mkaravel/feature/new_cross_track_strategy

Feature/new cross track strategy