Cascade PID Controller Implementation (#554)

* introduce an inverse dynamics (ID) controller. We select between PID vs ID custom controllers based on the actuator_user field that is defined per actuator. `user="1"` would trigger activating an ID controller where the gainprm are interpreted as Kp, Td, ema_smoothing_term, respectively.

* renaming

* Clean

* Cast to int

* wip cascade

* Enabled PID and cascaded PI control

* refactored PIDs

* fix id

* add some comments

* some more formatting

* test tweaks and fix to apply gravity compensation pre-clamping

* updated cascade documentation

* rm unused error term, move effort limit definition closer to where its used

* remove unused memory allocation

* simplify des_vel definition

Co-authored-by: rubendsa <ruben@openai.com>