Add long pixels corection to masking

Modify MaskCreator to correct for higher occupancy that is to be expected in larger pixels. This also involved adding a getPixelArea() function to the various Detector implementations, and should also help for using this module with the polar detectors (although the implementation might need a look-over).

I've also added a square_big_pixel_weight config option - in our detectors we get a lot of cross-talk between the big pixels so the occupancy increases ~as the square of the area. Probably not very globally applicable, but we need it for some datasets - I can remove it again if that would be desirable.