Tweak find_xyz_cut_coords to take an niimg as argument and return world (rather than voxel) coordinates

Fixes #234 


Have to use such a high tolerance to get the test to pass. Here is the error with rtol=4e-2

Not equal to tolerance rtol=0.04, atol=0

(mismatch 100.0%)
x: array([ 49.5, 9.5, 39.5])
y: array([50, 10, 40])