Application bug fixes and performance enhancements

[scottwittenburg]

This PR fixes several issues with the application:

• Use itk.js WorkerPool to speed up loading
• Fix issues encountered when interrupting currently loading experiment
• Support export of original CSV format
• Work around issue using older python 3.5 until we can upgrade python in the docker image stack
• Fix load state indicators:
  • Previously scans were marked as loaded or not loaded using filled vs. empty disc next to each scan. The computation for this was done efficiently, but was ultimately incorrect as the scan was marked as loaded as soon as a worker had picked up the last image to load. In cases where the only image in a scan is really large, the scan could be marked as loaded before it was actually ready.
  • This change removes the filled circles indicating individual scans are loaded or not in favor of a single icon located next to the experiment name.

Closes #18. Closes #42.

[dzenanz]

This dragged in all the commits from #35, because it has not been merged yet.

[scottwittenburg]

Yes, thanks @dzenanz, if this becomes good enough to consider truly adding, I'll either tack it on to the end of that PR, or else rebase this on master (after that's merged).

[dzenanz]

Time to rebase :D

[scottwittenburg]

This will need the cancel api addition to the WorkerPool in this PR to itk.js when it is merged and released.

[aashish24]

itk-js + miqa = big smile :)

[scottwittenburg]

@jeffbaumes This is the PR I was just mentioning