SolidMatrix: Fix dense()
TriMesh: Add face_texcoords and set_face_texcoords

Also make add_mesh propagate halfedge texcoords.
force/force_test: Check definiteness fixing

1. Check definiteness of elastic gradient in force_test.
2. Fix forces which don't implement or partially implement definiteness
   fixing to throw NotImplementedError, so that the unit tests can be
   automatically disabled.
3. Switch SimpleShell construction to take Dm.
4. Fix typo in SimpleShell definiteness fixing.