Setup of boundary cells
Cells distace over face

class UnstructuredMesh has no specializations now

I realized, that it would be better to implement methods working with mesh elements in the class UnstructuredMesh.
Finally, I managed to implement concept of unstructured mesh in any dimension.
Further work will be done after consultations.

Now the intent is clear and only few things like center points are to be implemented.
Then create wrappers for cells, faces etc.
Also is necessary to create methods for calculating lebesgue meassure.