Increase epsilon in MultiLinearSpanInterpolation test

[knelli2]

Proposed changes

Found from this CI, increase the epsilon. First commit doesn't change the test at all, just makes formatting conform to our standards.

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Further comments

[nilsdeppe]

LGTM, did you want to do the last few clang-tidy things? Up to you :)

[nilsdeppe]

looks like clang-tidy wants these using gsl::at too (or some subset of them at least)

[knelli2]

Oops, my bad. Squashed those in as well.

[nilsdeppe]

Thank you!!! :D