Add Akima method, get monotonic methods to work in nd

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 63.43284% with 49 lines in your changes are missing coverage. Please review.

Project coverage is 91.56%. Comparing base (64b6b34) to head (284a70c).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #23 +/- ## ========================================== - Coverage 97.72% 91.56% -6.16% ========================================== Files 6 6 Lines 658 759 +101 ========================================== + Hits 643 695 +52 - Misses 15 64 +49 ``` | [Files](https://app.codecov.io/gh/f0uriest/interpax/pull/23?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rory+Conlin) | Coverage Δ | | |---|---|---| | [interpax/\_spline.py](https://app.codecov.io/gh/f0uriest/interpax/pull/23?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rory+Conlin#diff-aW50ZXJwYXgvX3NwbGluZS5weQ==) | `97.73% <100.00%> (ø)` | | | [interpax/\_fd\_derivs.py](https://app.codecov.io/gh/f0uriest/interpax/pull/23?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Rory+Conlin#diff-aW50ZXJwYXgvX2ZkX2Rlcml2cy5weQ==) | `76.07% <62.59%> (-23.00%)` | :arrow_down: |