Removing state pollution in init_ExpStock.__next__()

[sturmianseq]

This PR aims to improve test reliability of test test_append_param by removing state pollution in init_ExpStock.__next__() by making a deep copy.

The test can fail in this way by running pip3 install pytest-repeat; python3 -m pytest --count=2 expstock/test_expstock.py::test_append_param:

    def test_append_param(init_ExpStock):
        e = init_ExpStock.__next__()
        test_param1 = 12345
        test_param2 = 'test_param'
        e.append_param(test_param1=test_param1, test_param2=test_param2)
        e.params.sort()
>       assert e.params == [('test_param1', 12345), ('test_param2', 'test_param')]
E       AssertionError: assert [('test_param1', 12345),\n ('test_param1', 12345),\n ('test_param2', 'test_param'),\n ('test_param2', 'test_param')] == [('test_param1', 12345), ('test_param2', 'test_param')]

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.