[assign] Add variant support
[assign] Fix compiler errors
Merge remote-tracking branch 'boostorg/geometry/develop' into variant/assign
[assign] Add tests for variants
[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.
[assign] Move concept check
Merge pull request #37 from sdebionne/variant/assign

[assign] Add variant support