New features

• The actions agent.action.u (which before corresponed to agent.state.force) are now detached from the physics. Dynamics will read actions and write into agent.state.force and agent.state.torque. This allows users to play around with action sizes and new dynamics more flexibly.
• At agent construction you can specify the agent dynamics (Agent(dynamics=Holonomic()) is the default). There are many available (Holonomic,HolonomicWithRotation,DiffDrive,KinematicBicycle)
• The dynamics will define the action size. You can also override this and define an action size yourself Agent(action_size=3). But you have to provide at least the minimum number of actions required by the dynamics (dynamics.needed_action_size). The dynamics will use the first dynamics.needed_action_size actions and the rest can be used for other user needs
• The discrete actions will have 3 options for each agent.action_size action. By default the holonomic agent has 2 actions: force_x (with 3 options stay,decrement,increment) and force_y (with 3 options stay,decrement,increment). Holonomic agent with rotations have 3 actions, with the additional one controlling torque (with the 3 options stay, decrement, increment). And so on for other dynamics....
• Action ranges, multipliers and noise can be controlled at agent construction with u_range, u_multiplier and u_noise. These can either be a scalar or a list. In case of a list, the list must have length agent.action_size

BC-BREAKING Changes

• Removed u_rot_range,u_rot_multiplier in favor of a more general u_range, u_multiplier and u_noise which can be a list (one elem per action )
• Removed MultiDiscrete actions. While before in case of multiple actions there was a space Multidiscrete([action_1_size, action_2_size]) , now there is a space DIscrete(action_1_size * action_2_size). It is still possible to ask MultiDIscrete action spaces by setting vmas.make_env(multidiscrete_actions=True)
• Discrete actions for the default dynamics (holonomic no rotation) have changed from 5 (up,down,left,right,stay) to 9, that now include the corners.

FIxes #51
Fixes #75

cc @Jianye-Xu @Zartris