FWD/BWD_OPT do not handle NACs correctly

[NilsWeidmann]

In CC and CO, it is sufficient to check the input models for NACs because they are already complete and the correspondence model does not contain any NACs. However, NACs on the target side cannot be considered in FWD_OPT and NACs on the source side in BWD_OPT vice versa.
An idea would be to encode application conditions in ILP constraints.