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/user/user_model.h

@@ -218,6 +218,7 @@ class mjCModel {
   int ntupledata;                 // number of objects in all tuple fields
   int nnames;                     // number of chars in all names
   int nM;                         // number of non-zeros in sparse inertia matrix
+  int nD;                         // number of non-zeros in sparse derivative matrix
 
   //------------------------ object lists
   // objects created here