Adding table unique method

This PR adds the ``table.unique`` method following the suggestions in #3001. There is place for improvements as masked columns are not checked for uniqueness, however I think it would be nice to have this in for 1.0 even with this limitation.
Tests, docs and changelog is added. One trivial commit containing the removal of unused imports from other table test is also included, however I can remove it if that is preferred.

cc :@taldcroft

closes #3001.