strel: implement SE decomposition for the octagon shape.
* @strel/getsequence.m: implement sequence to perform SE decomposition
of an octagon shape.
* @strel/strel.m: change input check for the octagon shape to be a
non-negative multiple of 3 (so it also accepts a value of 0), and
implement that special case. Convert unecessary %!shared block into
different %test blocks. Add more tests for the octagon shape.