Removing state pollution in zones

[sturmianseq]

This PR aims to improve test reliability of test test_addobject by removing state pollution in python list zones by calling method del

The test can fail in this way if state pollution in zones is not removed:

        """py.test for ex_addobject.py"""
        zones = bunchdt["zone"]  # all the zones
>       assert len(zones) == 7
E       assert 8 == 7