Unify constants for electron mass

Description

In many files there were two identical constants for the electron mass. The duplication is removed and unused imports are deleted.

Closes #179

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[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Documentation (update in the documentation)
[x] Internal refactoring

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[ ] My change requires a change to the documentation.
[ ] I have updated the tests accordingly (required for a bug fix or a new feature).
[ ] I have updated the documentation accordingly.
[x] I have reformatted the code and checked that formatting passes (required).
[x] I have have fixed all issues found by flake8 (required).
[x] I have ensured that all pytest tests pass (required).
[ ] I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
[x] I have checked that the documentation builds (required).

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