merge_partitions: add optional keyword original_topology

[Huite]

The current merging of partitions is opportunistic in terms of face ordering, e.g. simply concatenating. This means that the resulting merged topology might have a different face numbering than the original topology, pre-partitioning.

Then, the results cannot be compared with e.g. input: while the topology is equivalent, the ordering of values is different.

[Huite]

Solved by f02cdcb84b03b4e9acfa7d5f1a5c30bab6f0142b