Add Finite Strain Viscoelastic Material Formulation

[adtzlr]

• [x] as template material models.Viscoelastic(mu, eta, dtime) and
• [x] the material formulation models.finite_strain_viscoelastic(mu, eta, dtime).

closes #126

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (1ac836d) compared to base (5390b66). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #127 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 21 22 +1 Lines 786 794 +8 ========================================= + Hits 786 794 +8 ``` | [Impacted Files](https://codecov.io/gh/adtzlr/matadi/pull/127?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler) | Coverage Δ | | |---|---|---| | [matadi/\_\_about\_\_.py](https://codecov.io/gh/adtzlr/matadi/pull/127/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler#diff-bWF0YWRpL19fYWJvdXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [matadi/models/\_\_init\_\_.py](https://codecov.io/gh/adtzlr/matadi/pull/127/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler#diff-bWF0YWRpL21vZGVscy9fX2luaXRfXy5weQ==) | `100.00% <100.00%> (ø)` | | | [matadi/models/\_templates.py](https://codecov.io/gh/adtzlr/matadi/pull/127/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler#diff-bWF0YWRpL21vZGVscy9fdGVtcGxhdGVzLnB5) | `100.00% <100.00%> (ø)` | | | [matadi/models/\_viscoelasticity.py](https://codecov.io/gh/adtzlr/matadi/pull/127/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler#diff-bWF0YWRpL21vZGVscy9fdmlzY29lbGFzdGljaXR5LnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andreas+Dutzler)

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