add axis = 0 for nansum

[veni-vidi-vici-dormivi]

Fix for mistake made in #433. nansum needs an axis to sum along, otherwise it will just sum over all dimensions and output a single number.

[codecov[bot]]

Codecov Report

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

Project coverage is 81.94%. Comparing base (9b0b76b) to head (c4c74c1). Report is 36 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #445 +/- ## ========================================== - Coverage 87.90% 81.94% -5.96% ========================================== Files 40 44 +4 Lines 1745 1939 +194 ========================================== + Hits 1534 1589 +55 - Misses 211 350 +139 ``` | [Flag](https://app.codecov.io/gh/MESMER-group/mesmer/pull/445/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/445/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MESMER-group) | `81.94% <ø> (-5.96%)` | :arrow_down: | 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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