Show a warning when GroupWithGenerators called on a group

In GAP <= 4.9, the following undocumented use of GroupWithGenerators
was possible:

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

This does not work in GAP 4.10.0, and #3095 restored this never documented
behavior to avoid regressions in code that used to work previously.

This commit adds a warning when such use of `GroupWithGenerators` is detected.

lib/grp.gi

@@ -4450,6 +4450,13 @@ InstallMethod( GroupWithGenerators,
 function( gens )
 local G,typ;
 
+ if IsGroup(gens) then
+ Info( InfoPerformance, 1,
+ "Calling `GroupWithGenerators' on a group usually is very inefficient.");
+ Info( InfoPerformance, 1,
+ "Use the list of generators of the group instead.");
+ fi;
+
  gens:=AsList(gens);
  typ:=MakeGroupyType(FamilyObj(gens),
  IsGroup and IsAttributeStoringRep 
@@ -4468,6 +4475,13 @@ InstallMethod( GroupWithGenerators,
 function( gens, id )
 local G,typ;
 
+ if IsGroup(gens) then
+ Info( InfoPerformance, 1,
+ "Calling `GroupWithGenerators' on a group usually is very inefficient.");
+ Info( InfoPerformance, 1,
+ "Use the list of generators of the group instead.");
+ fi;
+
  gens:=AsList(gens);
  typ:=MakeGroupyType(FamilyObj(gens),
  IsGroup and IsAttributeStoringRep