add functions for controlling global parallelism settings

[codecov[bot]]

Codecov Report

Patch coverage: 35.29% and no project coverage change.

Comparison is base (e94ea9a) 92.01% compared to head (c0b94eb) 92.02%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #60 +/- ## ======================================= Coverage 92.01% 92.02% ======================================= Files 49 49 Lines 52551 52567 +16 ======================================= + Hits 48357 48373 +16 Misses 4194 4194 ``` | [Files Changed](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah) | Coverage Δ | | |---|---|---| | [faer-core/src/matrix\_ops.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9tYXRyaXhfb3BzLnJz) | `90.80% <ø> (ø)` | | | [faer-core/src/lib.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9saWIucnM=) | `68.56% <35.29%> (-0.17%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/60/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah)

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