Improve writing to storage in multiprocessing environment

[codecov[bot]]

Codecov Report

Base: 87.73% // Head: 87.71% // Decreases project coverage by -0.01% :warning:

Coverage data is based on head (41978de) compared to base (f90adca). Patch coverage: 53.33% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #325 +/- ## ========================================== - Coverage 87.73% 87.71% -0.02% ========================================== Files 69 69 Lines 10379 10388 +9 ========================================== + Hits 9106 9112 +6 - Misses 1273 1276 +3 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `87.71% <53.33%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/zwicker-group/py-pde/pull/325?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group) | Coverage Δ | | |---|---|---| | [pde/storage/file.py](https://codecov.io/gh/zwicker-group/py-pde/pull/325/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL3N0b3JhZ2UvZmlsZS5weQ==) | `84.50% <53.33%> (-1.37%)` | :arrow_down: | | [pde/solvers/controller.py](https://codecov.io/gh/zwicker-group/py-pde/pull/325/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=zwicker-group#diff-cGRlL3NvbHZlcnMvY29udHJvbGxlci5weQ==) | `93.47% <0.00%> (+0.72%)` | :arrow_up: | 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=zwicker-group). 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=zwicker-group)

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