Harmonic model return residuals

[veni-vidi-vici-dormivi]

• [x] Closes #459
• [x] Tests added

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.79%. Comparing base (9b0b76b) to head (b9efe99). Report is 90 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #460 +/- ## ========================================== + Coverage 87.90% 88.79% +0.88% ========================================== Files 40 43 +3 Lines 1745 1883 +138 ========================================== + Hits 1534 1672 +138 Misses 211 211 ``` | [Flag](https://app.codecov.io/gh/MESMER-group/mesmer/pull/460/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MESMER-group) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/MESMER-group/mesmer/pull/460/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MESMER-group) | `88.79% <100.00%> (+0.88%)` | :arrow_up: | 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=MESMER-group#carryforward-flags-in-the-pull-request-comment) to find out more.

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[veni-vidi-vici-dormivi]

Do you know if the tests are faster because they abort earlier or because the fitting is genuinely faster?

Should be because they are generally faster because also before they aborted at low orders (for few time steps).

[mathause]

Yes you are right it seems to be both :champagne:

[mathause]

This brings down the time of the tests by 30 s (1/3) - another nice win :1st_place_medal: