Free body order, new disk effect and Python 3 compatibility

• Python 2 and 3 compatible
• Major refactor that changes the way cases are defined in Python
• Most massive body is not required to be the first one anymore
• WHFast will use the most massive body as the central star independently of where it is in the list of bodies
• The central body for tidal/rotational flattening effects can be a different one from the most massive body
• Tidal and rotational flattening effect should have the same central body
• The central body for general relativity effects should still be the most massive body
• Effects are automatically turned off if there is no particle affected by it (e.g., tides are off if no central body for tidal effects is defined)
• Added disk effect, which can assigned to any body and not only the first one
• Added option to enable/disable disk interactions for non-hosts
• Added benchmark tests to measure execution speed changes
• Added --limit argument to stop the simulation after N seconds (useful for computer jobs with time limits)
• Added --time-limit resume argument to change the simulation time limit (useful to keep simulation a promising previous short simulation)
• Added compilation-time option to do compensated summation for gravity calculation
• Bugfix: Leconte2011 radius of gyration was not being read from the right column

The same input file as v2019.07.30 can and should be used with this version.

Improved integrators

• Improved spin integration in WHFAST with compensated summation
• Print snapshot filename when a simulation ends
• Added tests to python and rust packages
• IAS15 optimization: dynamic vector to fixed arrays
• Spin integration included in IAS15 schema
• Added Kidder 1995 2nd order post newtonian and spin-orbit
• Dynamically restore any integrator
• Bugfix: corrected IAS15 factors computation
• Bugfix: Corrected computation for the conservation of the angular momentum
• Bugfix: IAS15 time step computation and radius/evolution

Bugfixes for midpoint and democratic helio/WHDS integrations

• Corrected midpoint integrator which was not correctly computing the second part of the integration (it was not using positions and velocities increased by half step)
• Midpoint limited to gravitational and additional accelerations
• Removed the central body acceleration addition to planets (inertial) when using democratic heliocentric or WHDS (not necessary, verified with Mercury-T and Posidonius with all coordinate system tests)
• Additional accelerations are computed and added directly to the inertial system

The same input file as v2017.12.04 can and should be used with this version.

Dependencies upgraded

This release updates all the libraries and migrates to serde as parser/serializer since the previous library stopped working with rustc >= 1.23.

The same input file as v2017.12.04 can and should be used with this version.

First public release

This is the first public release of Posidonius, a N-body code for simulating planetary and/or binary systems.