fix NaNs handling in generate_wind_directional_kernels() (#323)

* fixing kernel positioning

* adding direction re-ordering

* make gaussian filter a touch bigger

* adding artifact for likely internatl documentation about effect of kernel orientation

* fix: correct typo in docstring for generate_wind_directional_kernels function

* add fix for NaNs in convolution

* adding nans back in at end, updating threshold for assignment to zero, adding docs

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Co-authored-by: Oriana Chegwidden <oriana@carbonplan.org>

Author: andersy005