integrate area/volume within a mask

So once #163 is solved, it would be very useful (particularly in the realm of Images) to integrate an interpolation within a shape. This is useful if we interpolated an image, and we want to find the sum of the image within a mask of some shape. Normally, you just sum all the pixels, but with this interpolation the integration would be highly accurate, no matter how sparse the pixel array is.

Obviously a feature...