Crocoddyl Actuation Model with External Forces

[EpicDuckPotato]

I posted an issue in pinocchio recently where my robot contains thrusters and revolute joints, where the relative placement of the thrusters with respect to the base frame varies based on the joint angles.

stack-of-tasks/pinocchio#1513

The conclusion was that I needed to treat the thrusters as external forces in the individual joint frames, then compute dtau_dx in my actuation model using pinocchio's computeABADerivatives. It would be helpful if there were an actuation model predefined where some actuators map to external forces on the robot.