Allow CQMs to be constructed in storage rather than memory

[arcondello]

For very large CQMs, it would be helpful to have something like numpy.memmap. It would wrap a CQM file, as generated by CQM.to_file. Constraints and objectives could then be added directly to storage rather than being stored in memory.

[arcondello]

Changed the title because memory-mapped is not quite what we're talking about here.