Rebound

* Fixes an issue with the WebGL widget.
* Fixes an issue with external forces and MERCURIUS.

* MERCURIUS is not compatible with binary files and the Simulationarchive.

* The WHFast integrator now supports Jacobi coordinates (default), democratic heliocentric coordinates and WHDS coordinates. The previously separate WHFastHelio integrator has been removed. The coordinate system can now be changed by simply setting the coordinates flag in the ri_whfast struct.
* Included an experimental new integrator MERCURIUS. This is similar to the hybrid integrator in Mercury but uses WHFast and IAS15. Not ready for production yet.

* Added a screenshot functionality for the WebGL ipython widget. This lets you take screenshots programmatically which is useful to create movies of simulations.

* Removed the march=native compiler flag as it seems to be problematic for some OSX/Sierra compilers.

* 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.