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.