Better initialization value for prune table

[jakobkogler]

In group.cpp use a better initialization value for the prune table:

prune_table = vector<int>(state_count, 20);

Maybe pass a reasonable value to the constructor or to the method call. In that case, the search doesn't need to go until:

visited_count < state_count

The loop could look like this:

for (int depth = 0; depth < max_depth; depth++)