add option to customize layout function for export

hetpy/models/hetGraph.py

@@ -447,16 +447,18 @@ def get_edges_of_type(self, type : str) -> List[Edge]:
 
 
     # util function for graph file storage
-    def export_to_json(self, filepath: str):
+    def export_to_json(self, filepath: str, layout_function='auto'):
         """
         Exports the graph to a json file to share or expose.
 
         Parameters:
         --------------
             filepath : str
                 The filepath where the resulting json data shall be stored.
+            layout_function : str
+                The layout after which the nodes should be positioned. All iGraph layout functions are valid. Defaults to 'auto'.
         """ 
-        layout = self.graph.layout(layout='auto')
+        layout = self.graph.layout(layout=layout_function)
         edges_json = []
         for index, edge in enumerate(self.edges):
             edge_dict = {e: getattr(edge, e) for e in dir(edge) if not e.startswith('__') and e != "nodes"}