Add shell plugin.

- Add bending passive force
- Add two examples (plate and poncho)
- Add stretching constraints for particles using tendons on mesh edges

PiperOrigin-RevId: 566980584
Change-Id: Id0cd8e1084c05560f1008518a8afbd5311d563e2

model/plugin/elasticity/plate.xml

@@ -0,0 +1,54 @@
+<!-- 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="Plate">
+  <include file="scene.xml"/>
+
+  <extension>
+    <plugin plugin="mujoco.elasticity.shell"/>
+  </extension>
+
+  <option solver="CG" tolerance="1e-6"/>
+
+  <size memory="10M"/>
+
+  <visual>
+    <map stiffness="100"/>
+  </visual>
+
+  <default>
+    <default class="wall">
+      <geom type="plane" size=".5 .5 .05"/>
+    </default>
+  </default>
+
+  <worldbody>
+    <body mocap="true" pos="-.1 .05 0" zaxis=".5 0 1">
+      <geom type="capsule" size=".1 .1" group="1"/>
+    </body>
+    <composite type="particle" count="16 16 1" spacing="0.05" offset="0 0 1">
+      <geom size=".02" rgba=".8 .2 .1 1" group="4" density="1000"
+            condim="3" solref="0.01 1" solimp=".95 .99 .0001"/>
+      <skin inflate="0.01" rgba=".4 0 .3 1" subgrid="4"/>
+      <plugin plugin="mujoco.elasticity.shell">
+        <config key="poisson" value="0"/>
+        <config key="damping" value="0.001"/>
+        <config key="thickness" value="1e-2"/>
+        <!--Units are in Pa (SI)-->
+        <config key="young" value="3e7"/>
+      </plugin>
+    </composite>
+  </worldbody>
+</mujoco>