Replace np.NAN with np.nan everywhere

[aekiss]

fixes #21

[anton-seaice]

Thanks @aekiss - it looks like the test dependencies also expect some numpy 1 features.

See

https://github.com/COSIMA/esmgrids/actions/runs/12782220423/job/35631353902?pr=22#step:5:97

We won't need the cice_from_mom step after the next ACCESS-OM3 release, so i suggest we just revert to numpy 1. We may not need this package going forward, otherwise well need to update the ocean_model_grid_generator to use numpy 2 before we use this.

Reverting to numpy 1 means the tests pass: #23

[aekiss]

hm, ok but np.NAN appears in many places in the code, so I expect there will be things other than cice_from_mom that break under numpy 2 - are we happy to stay on numpy 1 indefinitely?

[anton-seaice]

I think we won't really need this package once we stop using the CICE grid file (e.g. from the next OM3 release - 0.4.0) and its safe to assume itll work for a couple of years still with numpy 1

[anton-seaice]

Superseded by #23