Add support for exact calculations at array borders

[BENR0]

Is your feature request related to a problem? Please describe.
Currently the all variogram related textures do not give exact results at the array borders due to division by a higher number of neighbours then they actually have.

Describe the solution you'd like
Divide border pixel values by correct number of neighbours depending on window and lag size. A starting point is already implemented as neighbour_count in utils. This needs to be "daskified" since for big arrays the function would allocate huge arrays (see here for using block id in the function to be mapped).