[crosses] Add variant support
[assign] Add variant support
[intersection] Add variant support
[crosses] Fix compile errors
[assign] Fix compiler errors
[expand] Add variant support
Merge remote-tracking branch 'boostorg/geometry/develop' into variant/assign
Merge remote-tracking branch 'boostorg/geometry/develop' into variant/crosses
Merge remote-tracking branch 'boostorg/geometry/develop' into variant/expand
Merge remote-tracking branch 'boostorg/geometry/develop' into variant/intersection
[assign] Add tests for variants
[distance] re-factor segment-box code
[distance] eliminate more useless code in segment-box computation
[assign] Add copyright notice
[assign] Fix the variant tests

Reference was missing.
[assign] Rename the variant template parameters

Was T1 and T2 but that could eventually be a problem if the user defines
BOOST_VARIANT_LIMIT_TYPES > 10.
[crosses] Add tests for variants
[expand] Add tests for variant
[distance][segment-box] qualify all calls to get<I> with the geometry namespace
[distance][segment-box] eliminate unused variable (pp_strategy) in method call;
initialize result with 0;