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

include/mujoco/mjxmacro.h

@@ -112,6 +112,7 @@
     X( nuser_sensor )       \
     X( nnames )             \
     X( nM )                 \
+    X( nD )                 \
     X( nemax )              \
     X( njmax )              \
     X( nconmax )            \
@@ -508,6 +509,11 @@
     X( mjtNum,    efc_aref,          njmax,       1           ) \
     X( mjtNum,    subtree_linvel,    nbody,       3           ) \
     X( mjtNum,    subtree_angmom,    nbody,       3           ) \
+    X( int,       D_rownnz,          nv,          1           ) \
+    X( int,       D_rowadr,          nv,          1           ) \
+    X( int,       D_colind,          nD,          1           ) \
+    X( mjtNum,    qDeriv,            nD,          1           ) \
+    X( mjtNum,    qLU,               nD,          1           ) \
     X( mjtNum,    actuator_force,    nu,          1           ) \
     X( mjtNum,    qfrc_actuator,     nv,          1           ) \
     X( mjtNum,    qfrc_smooth,       nv,          1           ) \