Fix saving to file

[codecov-commenter]

Codecov Report

Base: 89.61% // Head: 89.71% // Increases project coverage by +0.10% :tada:

Coverage data is based on head (abaffb1) compared to base (43d587c). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #39 +/- ## ========================================== + Coverage 89.61% 89.71% +0.10% ========================================== Files 9 9 Lines 308 311 +3 ========================================== + Hits 276 279 +3 Misses 32 32 ``` | [Impacted Files](https://codecov.io/gh/dstansby/demcmc/pull/39?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby) | Coverage Δ | | |---|---|---| | [src/demcmc/dem.py](https://codecov.io/gh/dstansby/demcmc/pull/39?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby#diff-c3JjL2RlbWNtYy9kZW0ucHk=) | `90.36% <100.00%> (+0.11%)` | :arrow_up: | | [src/demcmc/tests/test\_mcmc.py](https://codecov.io/gh/dstansby/demcmc/pull/39?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=David+Stansby#diff-c3JjL2RlbWNtYy90ZXN0cy90ZXN0X21jbWMucHk=) | `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=David+Stansby). 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=David+Stansby)

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