define scalar simd api

[codecov[bot]]

Codecov Report

Attention: 191 lines in your changes are missing coverage. Please review.

Comparison is base (6305e8d) 91.87% compared to head (3752f06) 91.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #68 +/- ## ========================================== + Coverage 91.87% 91.91% +0.04% ========================================== Files 50 50 Lines 54908 54884 -24 ========================================== + Hits 50445 50448 +3 + Misses 4463 4436 -27 ``` | [Files](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah) | Coverage Δ | | |---|---|---| | [faer-cholesky/src/ldlt\_diagonal/compute.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGRsdF9kaWFnb25hbC9jb21wdXRlLnJz) | `95.06% <100.00%> (ø)` | | | [faer-cholesky/src/ldlt\_diagonal/update.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGRsdF9kaWFnb25hbC91cGRhdGUucnM=) | `88.57% <100.00%> (ø)` | | | [faer-cholesky/src/llt/compute.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L2NvbXB1dGUucnM=) | `92.46% <100.00%> (ø)` | | | [faer-cholesky/src/llt/update.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jaG9sZXNreS9zcmMvbGx0L3VwZGF0ZS5ycw==) | `87.52% <100.00%> (ø)` | | | [faer-core/src/householder.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9ob3VzZWhvbGRlci5ycw==) | `89.97% <100.00%> (ø)` | | | [faer-core/src/jacobi.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9qYWNvYmkucnM=) | `93.54% <100.00%> (ø)` | | | [faer-core/src/solve.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1jb3JlL3NyYy9zb2x2ZS5ycw==) | `97.83% <100.00%> (ø)` | | | [faer-evd/src/hessenberg.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1ldmQvc3JjL2hlc3NlbmJlcmcucnM=) | `99.80% <100.00%> (+6.31%)` | :arrow_up: | | [faer-evd/src/hessenberg\_cplx\_evd.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1ldmQvc3JjL2hlc3NlbmJlcmdfY3BseF9ldmQucnM=) | `97.99% <100.00%> (+0.04%)` | :arrow_up: | | [faer-evd/src/tridiag.rs](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sarah#diff-ZmFlci1ldmQvc3JjL3RyaWRpYWcucnM=) | `99.28% <100.00%> (+7.05%)` | :arrow_up: | | ... and [11 more](https://app.codecov.io/gh/sarah-ek/faer-rs/pull/68?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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