Limit the number of groups

[privefl]

It was a bit foolish of me, but I quickly tried running eulerr::euler() with 45 groups. This resulted in a crash. I guess the number of combinations scales very fast with the number of groups.

Maybe it would be a good idea to set a max number of groups (maybe as a parameter), and errors if the user inadvertently provides more.

[jolars]

Hm, you're right, some kind of check would probably be reasonable.