Better algorithm to decide if there is a differential or not.

[dalcde]

• Sometimes the potential differential target supports a non-trivial differential, so cannot be hit. We have to be careful, though, because it is possible that the differential supported is "fake" since the target is hit by a shorter differential
• Maybe make use of products, e.g. if the potential target is non-h_0 torsion but the class is h_0 torsion, then it cannot support any differential.