Show a warning when GroupWithGenerators called on a domain

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 gap-system#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

@@ -4401,6 +4401,13 @@ InstallMethod( GroupWithGenerators,
 function( gens )
 local G,typ;
 
+  if IsDomain(gens) then
+      Info( InfoPerformance, 1,
+          "Calling `GroupWithGenerators' on a domain usually is very inefficient.");
+      Info( InfoPerformance, 1,
+          "Use the list of generators of the domain instead.");
+  fi;
+
   gens:=AsList(gens);
   typ:=MakeGroupyType(FamilyObj(gens),
           IsGroup and IsAttributeStoringRep 
@@ -4419,6 +4426,13 @@ InstallMethod( GroupWithGenerators,
 function( gens, id )
 local G,typ;
 
+  if IsDomain(gens) then
+      Info( InfoPerformance, 1,
+          "Calling `GroupWithGenerators' on a domain usually is very inefficient.");
+      Info( InfoPerformance, 1,
+          "Use the list of generators of the domain instead.");
+  fi;
+
   gens:=AsList(gens);
   typ:=MakeGroupyType(FamilyObj(gens),
           IsGroup and IsAttributeStoringRep