Reset the data in mi to the initial state

[sturmianseq]

This PR aims to improve test reliability of test test_modify by resetting the data in mi to the initial state by calling method mi.modify

The test can fail in this way by running pip3 install pytest-repeat; python3 -m pytest --count=2 multiindex/tests/test_multiindex.py::test_modify:

    def test_modify(mi):
>       mi.modify('emp_id', 786, 666)
...
>       setattr(obj, indexed_by, new_value)
E       AttributeError: 'NoneType' object has no attribute 'emp_id'

It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state pollution.