Support for contact reading

[wpumacay]

Similar to the tests done for MuJoCo, we should have similar functionality that exposes the contacts from the bullet internal contact manifolds to the user. This is a key feature as early termination is required for most RL-tasks in locomotion, and we will need to check whether or not a body has touched the ground in order to terminate an episode.

Possible-solution: For an example of the low-level calls needed in Bullet, check the btCollisionWorld::debugDrawWorld method, in which they grab the manifolds and then the contacts. Still, we might need to wrap this for each agent such that when multiple agents are in the scenario, each one of them has the required information.