False negatives for check_order

[alehed]

Currently check_order returns a non-empty list denoting an error when the bgp configuration would be possible.

• Expected behavior: returning an empty list since the requirement is satisfiable.
• Actual behavior: It returns a non-empty list from compute_dags()
• Test case: test_simplified_possible_bgp in grigori's 'tests/test_bgpgen.py'

I'm opening issues so that I don't loose track of them and that there is finally an unambiguous way of talking about the different ones.