Add type annotations

[codecov-commenter]

Codecov Report

Merging #10 (169a2ae) into main (77f9ea3) will increase coverage by 0.01%. The diff coverage is 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10 +/- ## ========================================== + Coverage 96.80% 96.81% +0.01% ========================================== Files 4 4 Lines 626 628 +2 ========================================== + Hits 606 608 +2 Misses 20 20 ``` | [Files](https://app.codecov.io/gh/f0uriest/interpax/pull/10?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rory+Conlin) | Coverage Δ | | |---|---|---| | [interpax/\_fourier.py](https://app.codecov.io/gh/f0uriest/interpax/pull/10?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rory+Conlin#diff-aW50ZXJwYXgvX2ZvdXJpZXIucHk=) | `100.00% <100.00%> (ø)` | | | [interpax/\_spline.py](https://app.codecov.io/gh/f0uriest/interpax/pull/10?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rory+Conlin#diff-aW50ZXJwYXgvX3NwbGluZS5weQ==) | `97.00% <100.00%> (+<0.01%)` | :arrow_up: |