Add function for creating a "global" attraction point

This point won't show up in the graph itself and can not be interacted
with. Instead it only has the purpose to attract all nodes in the graph.
This can be useful if you have unconnected nodes and want to prevent
them from floating away.

Graph.lua

@@ -3,7 +3,6 @@ local Edge = require('Edge');
 
 local Graph = {};
 
--- TODO remove / replace LÖVE functions
 function Graph.new()
     local self = {};
 
@@ -12,7 +11,7 @@ function Graph.new()
     local edges = {};
     local edgeIDs = 0;
 
-    local center = Node.new( 'center ', love.graphics.getWidth() * 0.5, love.graphics.getHeight() * 0.5);
+    local attractionPoint;
 
     ---
     -- Creates a new graph based on the information passed via the table
@@ -47,6 +46,10 @@ function Graph.new()
         self:removeEdges( node );
     end
 
+    function self:addAttractionPoint( ncx, ncy )
+        attractionPoint = Node.new( 'center', ncx, ncy );
+    end
+
     function self:addEdge( origin, target )
         for _, edge in pairs(edges) do
             if edge.origin == origin and edge.target == target then
@@ -74,7 +77,10 @@ function Graph.new()
         end
 
         for _, nodeA in pairs( nodes ) do
-            nodeA:attractTo( center );
+            if attractionPoint then
+                nodeA:attractTo( attractionPoint );
+            end
+
             for _, nodeB in pairs( nodes ) do
                 if nodeA ~= nodeB then
                     nodeA:repelFrom( nodeB );