Compute moment of inertia for concave and boundary meshes. Resolve #338.

- Mesh inertias can now be computed exactly for well-formed (no holes) non-convex meshes.
- To activate this feature, set `<compiler exactmeshinertia="true">` (defaults to `false`). This default may change in the future.
- Added `<geom shellinertia="true/false">` (defaults to `false`). When true, geom inertia is computed assuming all the mass is concentrated on the surface, and `density` is interpreted as surface density (mass/area). Currently only mesh geoms are supported.

PiperOrigin-RevId: 464368395
Change-Id: I17afd99b9b221c9d24ae951f6e63d5c61ff89820

test/user/testdata/inertia_shell.xml

@@ -0,0 +1,11 @@
+<mujoco>
+  <compiler exactmeshinertia="true"/>
+  <asset>
+    <mesh file="cube.obj" name="hollow_cube"/>
+  </asset>
+  <worldbody>
+    <body>
+      <geom type="mesh" mesh="hollow_cube" density="1" shellinertia="true"/>
+    </body>
+  </worldbody>
+</mujoco>