Clarify which angle is needed for DirectionalBias

[adehecq]

The docstring at this line is not clear enough. My understanding is that we need to provide the angle with reference to the x axis, in clockwise direction. That's what I inferred from looking at the results of that line (clever way of not interpolating the data btw 😉), providing the wrong angle means one may look at across-track instead of along-track direction. Ii not clarified, I think it's more intuitive for most people to see it as the angle w.r.t. the y axis, because satellite images are typically only slightly rotated compared to that axis.