Implement MPLS checking

[matthieugouel]

Check https://arxiv.org/abs/1901.10156

We could try to infer MPLS tunnels from traceroute and tag it on the prefix table.
For a start, we could infer easy one, and possibly have a user flag on the measurement to specify the sending of additional probing (e.g. pings) as an attempt infer the most difficult ones.

Note that we already save MPLS labels in the results table. But we could add the column on the prefix table anyway with more information on the taxonomy ("explicit", "implicit", "opaque", "invisible")