Make return type Option<usize>

[jvdd]

Thought it would be useful to return None when there are NaNs in the array.

But this seems less useful / necessary than I thought as np.argmin returns (just as the current code base) the index of the NaN in the array ⬇️

>>> np.array([1, 2, np.nan, 4], dtype=float).argmin()
2

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So the only use-case for returning None would be;

• when the array is empty (instead of current assertion / panic)
• when skipping nans and only nans in array

The second use-case seems also less useful / necessary as np.argmin returns (just as the current code base) 0 ⬇️

>>> np.array([np.nan, np.nan], dtype=float).argmin()
0