schwarz: `bool isFullyPeriodic()` should not be a method inside the ccu mesh class

[fnrizzi]

we need to move out the check if a mesh object is fully periodic because it has no mening if a mesh is a sample mesh.
the mesh class should only contain methods that make sense for a generic disjoint set of cells.
If we need to query whether the mesh is periodic, we can do so using a free function that accepts a mesh object and uses its connectivity.