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

src/user/user_model.h

@@ -124,6 +124,7 @@ class mjCModel {
   bool fusestatic;                // fuse static bodies with parent
   int inertiafromgeom;            // use geom inertias (mjtInertiaFromGeom)
   int inertiagrouprange[2];       // range of geom groups used to compute inertia
+  bool exactmeshinertia;          // if false, use old formula
   mjLROpt LRopt;                  // options for lengthrange computation
 
   //------------------------ statistics override (if defined)