Refactor calc_centered_at_slices, axis-independent
Refactor calc_centered_at_slices, axis-independent

Update the arguments to calc_centered_at_slices() to reflect the fact
that it works regardless of the ordering of the axes, e.g.
'width, height' vs. 'height, width'. It only matters that the slices
match up.

Adjust centered_at() to reflect be compatable.

Test Plan:
$ ./meta/run_tests.sh
.. Run 'mel rotomap-edit' and zoom around ..

aevri/mel,aevri/mel