Add support for loading and saving OFF files

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (44804f2) 94.75% compared to head (0e9440a) 95.08%.

Files	Patch %	Lines
src/extra/off.jl	98.55%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #61 +/- ## ========================================== + Coverage 94.75% 95.08% +0.33% ========================================== Files 15 16 +1 Lines 781 854 +73 ========================================== + Hits 740 812 +72 - Misses 41 42 +1 ```

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