Add algorithm for Casimir Effect (#7141)

* Add algorithm for Casimir Effect

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* Fix the line length

* Fix the line length

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* Import math module and use Pi

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* Update doctest results

* from math import pi

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>