Fix some examples in the manifold module.

Fix some examples in the manifold module.

The second to last example was wrong: it set manifold ids on the
interior, but should not have. Otherwise, this commit:
1. uses a consistent initialization order
2. refers to SphericalManifold as 'manifold'
3. uses the relatively new Triangulation::set_all_manifold_ids function
4. explicitly remove the manifolds attached to the Triangulations so
that the effect of each manifold is clear.

to tidy things up.