add assertion test to symmetry permutation

[samwaseda]

And I just realised that the algorithm doesn't always work. Here I add a simple assertion test in order to be able to fail properly when this happens.

In order not to repeat the mistake, I keep this one open for 24h.

[coveralls]

Pull Request Test Coverage Report for Build 1560820691

• 13 of 14 (92.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 69.956%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyiron_atomistics/atomistics/structure/symmetry.py	13	14	92.86%

Totals
Change from base Build 1555113194:	0.01%
Covered Lines:	11521
Relevant Lines:	16469

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💛 - Coveralls