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_util_misc.c

@@ -657,7 +657,7 @@ void mju_printMatSparse(const mjtNum* mat, int nr,
                         const int* colind) {
   for (int r=0; r<nr; r++) {
     for (int adr=rowadr[r]; adr<rowadr[r]+rownnz[r]; adr++) {
-      printf("(%d %d): %.6f  ", r, colind[adr], mat[adr]);
+      printf("(%d %d): %9.6f  ", r, colind[adr], mat[adr]);
     }
     printf("\n");
   }