Rebound

* JANUS integrator added. This is a bit-wise reversible high-order symplectic integrator. At this time, it remains experimental. Details about this integrator will be published in an upcoming paper.

* Changes to the WHFastHelio integrator. This integrator now uses democratic heliocentric coordinates and a Hamiltonian splitted as proposed by Hernandez and Dehnen (2017), WHDS, which splits the Hamiltonian into three parts. It has the advantage that the integrator solves the two body problem exactly. It is not compatible with symplectic correctors, this functionality has been removed for WHFastHelio. For very high accuracy integrations of stable planetary systems, the WHFast integrator in Jacobi coordinated (and potentially symplectic correctors) should be better suited.

* Various minor bug fixes. Added pre-timestep modifications for REBOUNDx.

* Various minor bug fixes. One related to exact_finish_time=1.

* Added real-time interactive 3D visualizations using WebGL for Jupyter notebooks. This is an early release. Not everything might be working yet and new feature will be added to the widget class. To try it out, simply run `sim.widget()` in a Jupyter notebook. Note that you need to have ipywidgets installed and enabled.
* Minor changes to the Visualization backend. This should not have any consequences for users.

* Now stores the first characters of the current githash in binary files. This is helpful when trying to restart simulations from a binary file and making sure one uses the same version of REBOUND than in the original run. Currently, the git hash is not automatically compared when reloading a binary file. To view the githash, use e.g. hexdump. The hash appears between the first and second zero character in the first 64 bytes of the file.