Dynamically determine chunk size for background subtraction tool

[CameronFRWatson]

Recently wrapped a pixel-wise background subtraction tool: #64

Did not expose the --chunk-size parameter to Galaxy users, since it has to do with tuning memory/execution time, rather than any tool functionality. For now, we are just having the tool run with the default chunk size. If we end up seeing that this is inefficient for larger images, it might be good to dynamically set chunk size parameter based on input file size at the time of execution. This will likely be instance-specific though.