Add support for loading and saving MSH files

[codecov[bot]]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (f993f04) 94.96% compared to head (20e647d) 94.09%.

Files	Patch %	Lines
src/extra/msh.jl	86.11%	15 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #62 +/- ## ========================================== - Coverage 94.96% 94.09% -0.87% ========================================== Files 16 17 +1 Lines 854 966 +112 ========================================== + Hits 811 909 +98 - Misses 43 57 +14 ```

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