Performance improvements and new features

[scottwittenburg]

• flatten sessionTree data structure
• compute array range when images are loaded into memory (supporting improved performance)
• make sure single arrows don't skip past last (or first) time step to next (or previous() scan
• make sure single arrow buttons (and left/right arrow keys) don't skip any time steps
• press+hold single arrow buttons (and press+hold left/right arrow keys) scan as fast as possible through time steps
• redesign window level controls
• support loading original csv format as well as new json format
• migrate vue-utilities code into this repo as it seems now defunct

This set of changes requires changes in vtk.js (here) and vue-utilties (here).

The vtk.js change was merged into master and the vue-utilities code was moved into this repo.

[scottwittenburg]

As we discussed last week, I'm just going to merge this now.