sarr: a pedestrian array splitter

SARR splits a 2D array into adjacent subarrays
in COL -> ROW order.

the subarrays are collected in a 3D array
of size [SR,SC,SP], which facilitates
block processing.

the engine exclusively uses RESHAPE and PERMUTE
to keep memory requirements low.

SARR ends with an error if the size of the subarrays
does not fit into the size of the array.

see
> help sarr
for help and an example for this self-explanatory snippet