Change GroupByGenerators to accept collections again

In GAP <= 4.9, this worked:

    gap> GroupByGenerators( Group( (1,2) ) );
    Group([ (), (1,2) ])

In GAP 4.10, this was broken and instead lead to a "method not found" error.
While strictly speaking this was never documented behavior, we restore it
to avoid regressions in code that relied on this undocumented behavior.

Resolves gap-system#2703

lib/grp.gi

@@ -4401,6 +4401,7 @@ InstallMethod( GroupWithGenerators,
 function( gens )
 local G,typ;
 
+  gens:=AsList(gens);
   typ:=MakeGroupyType(FamilyObj(gens),
           IsGroup and IsAttributeStoringRep 
           and HasIsEmpty and HasGeneratorsOfMagmaWithInverses,
@@ -4418,6 +4419,7 @@ InstallMethod( GroupWithGenerators,
 function( gens, id )
 local G,typ;
 
+  gens:=AsList(gens);
   typ:=MakeGroupyType(FamilyObj(gens),
           IsGroup and IsAttributeStoringRep 
             and HasIsEmpty and HasGeneratorsOfMagmaWithInverses and HasOne,