Exact weight calculation for fractional mask.

[perrette]

The current code rasterizes a grid cell (down to 100 x 100 subdivisions) to calculate the weights.
Jan Volkholz and Christof Schoetz pointed to me that a tool exists that calculates exact weights based on theoretical calculations: exactextract. It also has an R implementation, and I see python bindings are in the works (at present I'd rather call the executable directly from subprocess). Consider using this after updating the vector data (#1).