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/user/user_flexcomp.h

@@ -40,6 +40,15 @@ typedef enum _mjtFcompType {
 } mjtFcompType;
 
 
+typedef enum _mjtDof {
+  mjFCOMPDOF_FULL = 0,
+  mjFCOMPDOF_RADIAL,
+  mjFCOMPDOF_TRILINEAR,
+
+  mjNFCOMPDOFS
+} mjtDof;
+
+
 class mjCFlexcomp {
  public:
   mjCFlexcomp(void);
@@ -73,6 +82,7 @@ class mjCFlexcomp {
   double inertiabox;              // size of inertia box for each body
   bool equality;                  // create edge equality constraint
   std::string file;               // mesh/gmsh file name
+  mjtDof doftype;                 // dof type, all vertices or trilinear interpolation
 
   // pin specifications
   std::vector<int> pinid;         // ids of points to pin