Implement --batch and --outdir switches to ostagraph.py that can produce multiple graphs from one sim stats load using instructions in a file

Loading the stats corpus takes quite some time, and this gets worse the more graphs we want to introduce.
Better to load just once and then produce multiple graphs from the same corpus.
Json file format will be detailed once this implementation is complete.
Currently can't control which log (set) a particular stat is graphed from, but this capability will come shortly.