Properly cd back to pwd on solver error

[FriesischScott]

By using cd in connection with a folder it will automatically go back to the current directory even when the Solver fails to run.

Closes #109

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 :warning:

Comparison is base (5017461) 98.83% compared to head (aed7162) 98.83%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ========================================== - Coverage 98.83% 98.83% -0.01% ========================================== Files 22 22 Lines 688 686 -2 ========================================== - Hits 680 678 -2 Misses 8 8 ``` | [Impacted Files](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/112?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf) | Coverage Δ | | |---|---|---| | [src/solvers/solver.jl](https://app.codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/112?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf#diff-c3JjL3NvbHZlcnMvc29sdmVyLmps) | `100.00% <100.00%> (ø)` | |

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