2.2.0 - Doc improvements + bugfix for cases requiring fixtures

• Improved documentation to explain why @fixture should be used instead of @pytest.fixture. Fixed #125

• Fixed ValueError: fixture is being applied more than once to the same function when two functions parametrized with the same cases were sitting in the same file. Improved robustness when cases require fixtures, in particular when parametrized test/fixture sits in a class or when several of them sit in a class/module. Fixed #126

See documentation page for details.