Add implicit integrator.

Added analytic derivatives of smooth (unconstrained) dynamics forces, with respect to velocities:
  - Centripetal and Coriolis forces computed by the Recursive Newton-Euler algorithm.
  - Damping and fluid-drag passive forces.
  - Actuation forces.

A new implicit-in-velocity integrator is implemented using the analytic derivatives. This integrator lies between the Euler and Runge Kutta integrators in terms of both stability and computational cost.

PiperOrigin-RevId: 450377010
Change-Id: Ie192b441876c22e732fb749333926f296e0a09cc

src/engine/engine_forward.h

@@ -55,6 +55,9 @@ MJAPI void mj_Euler(const mjModel* m, mjData* d);
 // Runge Kutta explicit order-N integrator
 MJAPI void mj_RungeKutta(const mjModel* m, mjData* d, int N);
 
+// fully implicit in velocity
+MJAPI void mj_implicit(const mjModel *m, mjData *d);
+
 
 //-------------------------------- solver components -----------------------------------------------