Add an API to retrieve a set of slices "in one go"

goal

add an API to retrieve several slices along different axis in the same sequence.

details

This PR only focus on the 'high levelAPI' and provide a common implementation for all volumes. Performances will be bad as for now it will load the entire volume to access slices.

But PR to come will redefine the protected function _get_slices_from_files to fine tune this behavior according to the file format.

related to #100 (closed)

API

Expected usage example

slices = volume.get_slices(((0, 2), (1, 50)))
for (axis, index), data in slices:
    ...