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

model/flex/softbox.xml

@@ -0,0 +1,39 @@
+<!-- Copyright 2021 DeepMind Technologies Limited
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+         http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+
+<mujoco model="Soft box">
+  <include file="scene.xml"/>
+
+  <option solver="CG" tolerance="1e-6"/>
+
+  <size memory="30M"/>
+
+  <asset>
+    <texture name="texsponge" type="2d" file="sponge.png"/>
+    <material name="matsponge" texture="texsponge" specular="0.3"/>
+  </asset>
+
+  <worldbody>
+    <body pos="0 0 1" name="body">
+      <freejoint/>
+      <geom size=".1" contype="0" conaffinity="0" group="4"/>
+      <flexcomp name="softbox" type="box" count="7 7 7" spacing=".04 .04 .04"
+                radius="0.01" dim="3" material="matsponge" dof="radial">
+        <contact internal="false" selfcollide="none"/>
+        <edge equality="true"/>
+      </flexcomp>
+    </body>
+  </worldbody>
+</mujoco>