Openpathsampling

Continuing clean-up and adding conveniences as we prepare for 1.0.

Some improvements new in this release:
- switch JSON library to `ujson` (582)
- `masses`, `n_degrees_of_freedom`, and `instantaneous_temperature` snapshot features (579, 589)
- renamings surrounding what used to be `live_visualization` (596)
- end checking for "safe imports" in CVs (599)
- improvements to the committor calculation (601)

In addition, we removed lots of unnecessary/outdated code, reorganized several sections for better modularity, cleaned up our examples, and gave the documentation a long-overdue update.

First public (pre)release of OpenPathSampling.

OPS is now feature-complete for 1.0, and mostly API-stable. While we clean up the code and check the corners for bugs on our way to 1.0, we'd like to encourage people to get familiar with us by trying our 0.9 release.