fix: nanargmin with all nan slice

[manuba95]

This fixes an issue in the DiscreteVoronoiModel. The model crashes when frame are all NaN because of the np.nanargmin() function. This is because of type stability of the return of that function as its supposed to return integers. See pydata/xarray#4481 and pydata/xarray#3884.
One easy fix for this would be to apply the np.nanargmin() only to slices that are not all NaN using np.where(). However, masking the hole array before the loop and catching the all NaN slices early might be a more elegant and better performing option.

[codecov]

Codecov Report

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Project coverage is 95.58%. Comparing base (e1de087) to head (5d10c39).
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##           develop     #134   +/-   ##
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  Coverage    95.58%   95.58%           
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  Files           47       47           
  Lines         3194     3194           
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  Hits          3053     3053           
  Misses         141      141           

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[manuba95]

This commit fixes an issue with the previous commit. Although the np.where() is supposed to handle the all-nan slices, the np.nanargmin() is still called upon the entire array, resulting in the same error. This is now fixed by masking the array.
Although it is possible to catch this edge case earlier (e.g. by masking the array before the initial for loop) I think this is currently the approriate solution as other unexpected user choices may result in all-nan slices in the closest_player_index (e.g. manipulation of the mesh grid).
The tests all run properly but I think they should be extended to at least the all-nan slice edge case. Also, there is currently no test for the plotting, which needed a fix as well.
Also, I think it can be up to debate, weather inactive players (or teams) should have a space controll value of 0 (as currently) or if it should be np.NaN.
Here, I would appreciate some feedback before continuing finalizing this PR.