[distance] Add variant support

Add variant dispatch layer to the distance algorithm
[distance] Add variant tests

No finished yet
[distance] First step to fix tests
Merge from upstream
[distance] Merge from upstream
[distance] Fix some typo and add more tests
[distance] Use transform_variant

Remove dependency to is_implemented. Update transform_variant to support
MPL sequence input.
[doc] Extended doc for WKT
[doc][wkt] update of conformance sections
[buffer] fix for update get_turns
[buffer] fix for changed name geometry_multi_within_coe to point_in_geometry
[rescale] fix for points / empty boxes
[buffer] rename m_rescale_policy to m_robust_policy
[test] Temporarily disable rescale test
Merge remote-tracking branch 'boostorg/geometry/develop' into develop
[index] Varian-based nodes used in the rtree by default.

This should fix the crash when the rtree is used in the shared memory reported in this thread: http://boost-geometry.203548.n3.nabble.com/rtree-crash-when-used-with-inter-process-td4026037.html
[distance] Support different variant type

Add support for different variant type as parameters
[index] Remove unneeded template keyword from member function call
[distance] Remove global is_implemented
Merge pull request #26 from sdebionne/develop

[distance] Add variant support