Deprecated function call in filter_remove_small_holes

[SamuelVilz]

In tile_extraction/filter.py, in line 286, sk_morphology.remove_small_holes is called with the parameter name min_size.
This probably worked for earlier versions of the sk_morphology function, but it now raises an error, as the function now expects the parameter area_threshold, which is probably equivalent with what min_size once meant.
-> filter.py needs to be adapted