Add LowIndexNormalSubgroups method.

[FriedrichRober]

We need a high-level function for the User to just get a list of normal subgroups and not that Lins_Graph datastructure.
Need to keep in mind, that this operation is already declared in polycylic

[FriedrichRober]

I'm very much in favor of distributing the package. Would it be possible to add a trivial function

LowIndexNormalSubgroups:=function(g,n)
local l;
  l:=LowIndexNormalSubgroupsSearchForAll(g,n);
  return List(List(l),Grp);
end;

to cater to users who just want a list of actual subgroups without the fancier features?

Originally posted by @hulpke in #50 (comment)