Add an implementation of the quadratic RS problem.

[damar-wicaksono]

• The two-dimensional reliability analysis problem is a variant of the classic RS problem with one quadratic term. The function is implemented from Waarts (2000).

This PR should resolve Issue #270.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 :tada:

Comparison is base (27c2be0) 98.80% compared to head (9a4d71c) 98.81%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #275 +/- ## ========================================== + Coverage 98.80% 98.81% +0.01% ========================================== Files 52 53 +1 Lines 1840 1856 +16 Branches 139 139 ========================================== + Hits 1818 1834 +16 Misses 11 11 Partials 11 11 ``` | [Impacted Files](https://app.codecov.io/gh/damar-wicaksono/uqtestfuns/pull/275?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damar+Wicaksono) | Coverage Δ | | |---|---|---| | [src/uqtestfuns/test\_functions/\_\_init\_\_.py](https://app.codecov.io/gh/damar-wicaksono/uqtestfuns/pull/275?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damar+Wicaksono#diff-c3JjL3VxdGVzdGZ1bnMvdGVzdF9mdW5jdGlvbnMvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | | | [src/uqtestfuns/test\_functions/rs\_quadratic.py](https://app.codecov.io/gh/damar-wicaksono/uqtestfuns/pull/275?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Damar+Wicaksono#diff-c3JjL3VxdGVzdGZ1bnMvdGVzdF9mdW5jdGlvbnMvcnNfcXVhZHJhdGljLnB5) | `100.00% <100.00%> (ø)` | |

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