Provide the ability to window the output of propagate

[andykee]

In its current implementation, the shape of the output from propagate is determined by the npix parameter and the center of the output will always be on-axis.

It may be desirable to adjust the location of the output by accepting a window parameter to specify the indices within the output canvas with shape = npix to return. In this way, a user could establish a 2000 x 2000 output array but only return the lower left quadrant by requesting

window = [slice(1000, 2000), slice(0, 1000)]