Hackathon: dask backend

[aschampion]

WIP as of the hackathon on this branch so it doesn't disappear.

TODO:

• Ensure working w/o dask installed
• Use bag or other dask data container rather than a giant list of delayed futures
• Re-add error handling
• NBLAST dask backend

[aschampion]

It turns out dask distributed works as a joblib backend (I thought it didn't, and only non-distributed dask did). That's likely a better, generic, community standard approach, and satisfies my feeling that this was inside-out. I may hack on that over the holiday.