3.0.0 - harmonization of ids and public API for cases info

• Major refactoring of the way ids and marks are generated and customized in fixture_union, @parametrize and @parametrize_with_cases. Now idstyle has a consistent behaviour across the board, ids and idstyle can work together correctly, @parametrize_with_cases and @parametrize have much better default values for ids, and many others. See documentation for details. Fixed #154

• New public API to manipulate information about a case function: copy_case_info, set_case_id, get_case_id, get_case_marks, get_case_tags, matches_tag_query, is_case_class, is_case_function. See API reference.

• Fixed default behaviour of idgen in @parametrize: it only defaults to AUTO when no fixture_ref are used in the argvalues.

See documentation page for details.