Turn NautyDense into a 'global function'

... to fix warnings when loading NautyTracesInterface and
digraphs together.

Author: fingolfin

gap/NautyGraph.gd

@@ -402,3 +402,5 @@ DeclareGlobalFunction( "CREATE_NAUTY_GRAPH_OBJECT" );
 DeclareGlobalFunction( "CREATE_NAUTY_EDGE_COLORED_GRAPH" );
 
 
+##
+DeclareGlobalFunction( "NautyDense" );

gap/NautyGraph.gi

@@ -436,7 +436,7 @@ end );
 
 
 #! TODO: document this
-BindGlobal( "NautyDense",
+InstallGlobalFunction( "NautyDense",
 function( source_list, range_list, n, is_directed, color_data )
     local graph;
     graph := NAUTY_GRAPH( source_list, range_list, n, is_directed );