Consider placing each matrix as a separate file in DSS

[hannes-ucsc]

For MVP, we produce one matrix.mtx.zip for each project. That file is placed in the DSS as part of the bundle and uploaded to the project assets bucket. It contains multiple MTX matrices, not just one. While we need a single object in the assets bucket so that there is a single file to download, we do have the liberty of including multiple files in the bundle.

The overall matrix ZIP should go into projects/$uuid/matrices.zip and the individual matrices should go into projects/$uuid/bundle/$basename.mtx.zip where basename is some transformation of the original GEO file path (see #45). Either just use the basename and drop the directories from the path OR replaces slashes in the path with double underscore. A third alternative would be to leave the slashes in the path but slashes in DSS bundle manifest entries are not tested well. We would have to test that and make sure that the CLI's download functionality does the right thing as well.