Allow cache to live outside of package directory

[ghing]

Define a setting that would allow fetched files to live somewhere outside of the openelex package directory. This would allow core to just be pip installed and to keep data anywhere on the filesystem.

The long-term benefit of this is that it would allow using something like riofs to store fetched files directly to S3.

That would also allow spinning up EC2 instances for distributed fetching/loading of bit/memory intensive results data.

[dwillis]

+1 as well.

[zstumgoren]

+1