Tests for triangles and tetrahedrons + fix 3d stress evaluation

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (35fc5a1) 89.83% compared to head (8869e81) 89.81%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #203 +/- ## ========================================== - Coverage 89.83% 89.81% -0.03% ========================================== Files 44 44 Lines 1908 1904 -4 ========================================== - Hits 1714 1710 -4 Misses 194 194 ``` | [Flag](https://app.codecov.io/gh/ikarus-project/ikarus/pull/203/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ikarus-project) | Coverage Δ | | |---|---|---| | [tests](https://app.codecov.io/gh/ikarus-project/ikarus/pull/203/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ikarus-project) | `89.81% <100.00%> (-0.03%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ikarus-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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[henrij22]

Todo for this PR

• [x] get rid of constexpr auto cubeGeometry and use cube(2) and don't have the 2 as template argument
• [x] merge the two checkStressSimplex Functions into one (dim-agnostic)
• [x] check for unused declarations
• [x] write here dim*(dim + 1) / 2 to directly support 3D
• [x] maybe use isApprox and better error displaying