Frederik Hennig authored 4 years ago and Markus Holzer committed 4 years ago

- `shift_slice` did not work for single slices as it always tried to
iterate its argument
- `shift_slice` returned lists of slices, but for accessing numpy
arrays, tuples of slices are required

The scheme corresponds to calculating the overlap volume of a cell shifted by the flow velocity with its neighbor cell. The new test does that explicitly, but generates convoluted expressions which should not be used directly because they are so slow to evaluate.

Thanks to this test, an incorrect sign in the implementation was found and fixed.