Use sheet2matrixidx for Image snapping

[philippjfr]

Currently snapping to the closest coordinate only reliably works when using the Image interface because there can be subtle floating point differences in the coordinates. Once we have an ndarray like indexing interface for gridded Datasets Image.select and Image.__getitem__ could generate integer indices and slices and apply them via this interface, bypassing any floating point issues in matching the coordinates.

[philippjfr]

Now closed, we now have efficient vectorized Image sampling, which works with all gridded interfaces.