Rebound

* Added new units shortcuts (year,years,massist)
* Rearranged some loops and switch statements (doesn't affect floating point numbers).

* Added pyproject.toml file

* Changed the way REBOUND reverses the integration direction when the sign of the timestep is inconsistent with respect to the requested final time.
* Fixes a memory leak when a tree code is used
* Fixes an issue where MERCURIUS was not bit-wise reproducible when safe mode was turned off.

* Minor changes to the python side of Vec3d to make it more compatible with numpy.

* Minor changes related to the REBOUND Rotations framework.

* Added the REBOUND Rotations framework.
* Fixes an issue with showing an incorrect periastron location in OrbitPlot for high mass-ratio systems.
* Adds pre and post timestep calls to the ode framework.