Starting on adding toy dynamics

The point of this branch is to make dynamics generators other than OpenMM
into feasible options.
Mainly commit-as-stash.
Merge remote-tracking branch 'origin/decouple_dynamics' into toy_dynamics
Merge remote-tracking branch 'origin/decouple_dynamics' into toy_dynamics
Merge remote-tracking branch 'upstream/master' into toy_dynamics
Merge remote-tracking branch 'upstream/master' into toy_dynamics
Gaussian and OuterWalls parts of Toy dynamics

Potential and tests. Also updates to correct interface for simulation object.
Toy PES completed, with tests.
Drafts of toy_integrators

Also renamed files to be a bit better for modules. Integrators are still
untested.
All tests have skeletons, plus some clean-up
Completed tests for integrators.
Tests pass; 100% coverage.
Added HarmonicOscillator. With tests.

Because everybody likes harmonic oscillators!
Added toy_dynamics_example.

(Which conveniently also serves as a test for the correctness of the
Langevin BAOAB integrator.)
Merge remote-tracking branch 'upstream/master' into toy_dynamics
Added langevin_integrator_check.ipynb.

Contains the stuff that was previously in toy_dynamics_example. Still plan
to add a few more things to it to make it prettier.
Improvements for langevin_integrator_check.ipynb

Now all the integrator tests are in there. A little more documentation and
this example is done.
langevin_integrator_check.ipynb looks complete.

Also caught some bugs in the harmonic oscillator PES (and tests thereof).
Removed the old toy_dynamics_example.py, since it has been superceded by the
ipynb.
Added toy_dynamics_tis.ipynb. Commit-as-stash.
Merge remote-tracking branch 'upstream/master' into toy_dynamics