Added spheres union (#6879)

* Spheres union

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* Update volume.py

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* Update volume.py

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* f-strings

* Update maths/volume.py

Co-authored-by: Christian Clauss <cclauss@me.com>

* more tests

* fix non negative

* fix 0 radius

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* fix tests

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* fix tests

* fix print

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* fix comment

* fix comment

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* Update volume.py

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