[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.
Merge remote-tracking branch 'boostorg/geometry/develop' into develop
[distance] Support different variant type

Add support for different variant type as parameters
[distance] Remove global is_implemented
Merge pull request #26 from sdebionne/develop

[distance] Add variant support