Use a manifold to generate better meshes for the shell.

This PR is, for now, intended mainly for discussion. I thought I'd give it a try to use a manifold description for the spherical shell. This works and produces nicer meshes indeed. But there are questions we need to answer first:

1/ It is not backward compatible. Meshes change from before, and that's going to break a lot of testcases.
2/ It requires a new enough deal.II version.
3/ It requires a bit of a hack at the boundaries because we need normal vectors there that the manifold does not provide. 

My original motivation was to see whether this helps with the issue we have with shell_simple_3d.prm. The answer is: in parts; the time step where it breaks moves from time step6 to time step 12.