v0.5.0

• Merges PR #1, #2

This allows to create many (different and non colliding) double pendulums as part of a single environment programmatically.
Multiple controllers can be wrapped into one controller which can control the many double pendulums environment.
For feasible reference trajectories the corresponding feedforward controller can be incorporated to visually create the moving reference pendulum position.

• Moves tree.py into seperate repository tree_utils