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

introspect/enums.py

@@ -118,6 +118,7 @@ ENUMS: Mapping[str, EnumDecl] = dict([
          values=dict([
              ('mjINT_EULER', 0),
              ('mjINT_RK4', 1),
+             ('mjINT_IMPLICIT', 2),
          ]),
      )),
     ('mjtCollision',