This reverts commit 8fb6f1d5.

This change makes the legacy writer to write UG with polyhedrons
that are corrupted and can't be loaded back.
Initial problem is that legacy format stores cell ids as int (32bits).
This change had several issues that should be addressed in the future.
We can at least mention those:
 - it only 'fixes' cases of meshes that containes some polyhedrons;
 - the reader is not able to read back the produced files;
 - by only changing the type, it keeps some loops that become useless.