AAG/AIG parser does not through error on acyclic ANDs

[zimmski]

The following property of the AIGER format is ignored even though it should throw an error:

Furthermore, the definitions of the ANDs have to be acyclic. To be
more precise: The literal on the LHS of an AND is defined to depend on the
literals on the RHS after stripping sign bits. The transitive non
reflexive closure of this dependency relation has to be acyclic.

Two examples that should fail are:

aag 3 0 0 0 2
2 5 4
4 3 0

aag 3 0 0 0 3
2 5 4
4 6 0
6 2 0