Pytest-cases

Fixed issue where fixtures get called with `NOT_USED` as a parameter when using a `fixture_union`. This issue is actually only fixed in `pytest_fixture_plus`, if you use `pytest.fixture` you have to handle it manually. Fixes [37](https://github.com/smarie/python-pytest-cases/issues/37).

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](https://docs.pytest.org/en/latest/proposals/parametrize_with_fixtures.html).

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.

Fixed `param_fixtures` issue: all parameter values were identical to the last parameter of the tuple. Fixes [32](https://github.com/smarie/python-pytest-cases/issues/32).

Following [Sup3rGeo](https://github.com/Sup3rGeo)'s proposal, introduced two helper methods to create simple "parameter fixtures". Fixes [#31](https://github.com/smarie/python-pytest-cases/issues/31).

`pytest_fixture_plus` now correctly honors parameter id and marks overridden at single parameter level using `pytest.param`. Fixed [30](https://github.com/smarie/python-pytest-cases/issues/30).

Fixed `pytest_fixture_plus` in case it is used with `parametrize` and one parameter is itself customized using `pytest.param`. Fixed [29](https://github.com/smarie/python-pytest-cases/issues/29).