Fix exponent clipping in Holland 2010 implementation

[tovogt]

Changes proposed in this PR:

• This PR is in response to a mail by a PhD student who had questions about the implementation of the Holland 2010 TC wind model in CLIMADA.
• The current implementation clips the hol_x exponent to the [0.0, 0.5] interval, even though this is not mentioned in the literature. I understand that negative values are undesirable because it would cause the wind speeds to increase outwards (which is unphysical). However, I don't think that clipping high values to 0.5 is necessary. That would just mean that wind speeds are decreasing very fast outwards, which seems okay to me.
• While nobody currently seems to use the Holland 2010 wind model (including me), it might still be interesting to ensure the correctness of the implementation.

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[peanutfun]

The actual change looks easy enough, but it's really hard for me to understand the tests. Could we maybe reduce the number of values we are testing against? 🤔

[peanutfun]

I am guessing the two zeros here are an effect of the clipping an something you wanted to explicitly test for?

[leonie-villiger]

In my understanding, the first zero results from the large distance between track and centroids (close_centr = False), thus does not test the clipping. The last zero tests the clipping by creating the conflict v_max_s < v_n (v_max_s = 12 and v_n defaulting to 17). To me, it is also unclear what is tested in the other two, added cases.

[tovogt]

Thanks for checking!

I modified the test cases to be more systematic, and also added explanations in the code.

[leonie-villiger]

Thanks for adapting the test. Your explanations make it is very clear what is tested. In my opinion, ready to merge.

[peanutfun]

This is much clearer now. Thank you, @tovogt! Merging.