Updates MeshGenSphericalShell
The mesh generator for thick spherical shells now also flags primitives using the MeshInfo::hollowFlag enum, like that for the full annulus does already. This means that vertices, edges and faces on the outer surface are marked as flagOuterBoundary, those on the inner as flagInnerBoundary and all others and of course cells as flagInterior.
The new HollowBodyUIDTest checks that this works correctly for both types of hollow bodies.