demo_zbigarray: Allow to specify work size as option
Since the beginning (1ee72371 "Demo program that shows how to work with ZBigArrays bigger than RAM in size") the work size was 2*RAM. However sometimes for demonstration purposes it could be handy to set it to be even more (e.g. 16*RAM) or vise versa - to something lower. So add command line option to do so instead of manually patching sources every time.
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