Remove unused temporaries in tridiagonal solver

[julianlitz]

Merge Request - GuideLine Checklist

Clean up the tridiagonal solver class. We currently supply a vector which can be used as a temporary. It can be removed, but maybe the kokkos solver would benefit from it?

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• Tests for new functionality has been added
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• No large data files have been added to the repository. Maximum size for files should be of the order of KB not MB. In particular avoid adding of pdf, word, or other files that cannot be change-tracked correctly by git.