HebiArmPlugin interface for custom HebiArm plugins
    
    Plugins have access to the arm state and may modify it as needed.
    Plugin specific state may also be stored in the state struct, 
    but users are responsible for avoiding variable name collisions.
 
    The always accessible fields are:
    * time [s]
    * dt [s] (time since last call, or 0 on first run)
    * fbk [struct]
    * cmdPos [rad]
    * cmdVel [rad/s]
    * cmdAccel [rad/s^2]
    * cmdEffort [Nm|N]