Use previous `LinsGraph` in next search

[FriedrichRober]

It would be nice to be able to use an already computed LinsGraph for the next search algorithm.

The most obvious use case would be that we computed all normal subgroups up to index n and now want to extend the lattice up to index n + k.

Of corse, we then need to think about how we manage our search options and how to signal where and why a certain branch was cut.

[FriedrichRober]

This would be especially interesting if Issue #37 is resolved.