gblk: a pedestrian data block extractor

GBLK computes index templates of the form
segment:offset
for very fast extraction of data blocks from 2d matrices of any type.

data blocks can be non-, partial-, or fully overlapping.

the output can be reused to extract data from other matrices of the same size.

the templates can be modified to extract only selected data blocks and/or to extract only selected values from a data block.

CELLFUN can be used to compute further results from the extracted data blocks

see
> help gblk
and the accompanying published M-file for info and a few self-explanatory examples.

an ND version of GBLK will be posted soon.