Introduce trilinear flex parametrization.

These flexes use only 24 DOFs (3 per vertex of the bounding box), while colliding with the full high resolution mesh.

On an 8x8x8 cube, the performance using DOFs at all vertices is

```
 Simulation time      : 18.74 s
 Steps per second     : 533
 Realtime factor      : 0.53 x
 Time per step        : 1874.4 µs

 Contacts per step    : 114.88
 Constraints per step : 3322.51
 Degrees of freedom   : 1536
```

With the new implementation, it is the following:

```
 Simulation time      : 1.82 s
 Steps per second     : 5507
 Realtime factor      : 5.51 x
 Time per step        : 181.6 µs

 Contacts per step    : 38.84
 Constraints per step : 155.36
 Degrees of freedom   : 24
```

PiperOrigin-RevId: 721008829
Change-Id: I833df027527db578d86667cc4b24295bcf6f7d22

src/engine/engine_util_misc.h

@@ -53,6 +53,11 @@ MJAPI mjtNum mju_muscleDynamics(mjtNum ctrl, mjtNum act, const mjtNum prm[3]);
 // all 3 semi-axes of a geom
 MJAPI void mju_geomSemiAxes(const mjModel* m, int geom_id, mjtNum semiaxes[3]);
 
+// ----------------------------- Flex interpolation ------------------------------------------------
+
+// evaluate the deformation gradient at p using the nodal dof values
+MJAPI void mju_defGradient(mjtNum res[9], const mjtNum p[3], const mjtNum* dof, int order);
+
 // ----------------------------- Base64 -----------------------------------------------------------
 
 // encode data as Base64 into buf (including padding and null char)