Use a DAG for territory aliases

[kdeldycke]

Territory code aliases implementation is quite a mess, and to make it easier to maintain, use a directed acyclic graph to store the subdivision / country relationship as well as aliases.

With such a structure, we can easily solve #5 and check for stupid cycles in unit tests.

I think I'll leverage the NetworkX package to do so.

[kdeldycke]

This might help solve once for all unsupported edge-cases. See: 5b589f8 and fe73027 .