Unify friction parameter

[maxspahn]

When torque control is used to control the robots, it is possible to set friction values for all joints.
Currently, there is a uniform friction value across all joints of one robot, for now, this is a sufficient solution.

However, the friction parameter must be accessible when initializing the environment. This also applies for the acceleration control environments that rely on inverse dynamics, such as the pandaReacher

The friction parameter should be an optional argument to the init function for acc.py and tor.py in the pandaReacher.
To be verified for other robots.