1.6.0 - `fixture_union` and `param_fixture[s]` bugfix

New fixture_union method to create a fixture that is the union/combination of other fixtures. This is an attempt to solve this pytest proposal.

Also, param_fixture and param_fixtures can now be used without necessarily storing the return value into a variable: they will automatically register the created fixtures in the calling module.

Finally, fixed a bug with param_fixtures when called to create a fixture for a single parameter.

See documentation page for details.