allow running enlarge on species that are already in core

this is exclusively for initial species that may be added to core during seed mech addition

Author: mjohnson541

rmgpy/rmg/model.py

@@ -635,7 +635,10 @@ def enlarge(self, new_object=None, react_edge=False, unimolecular_react=None, bi
                     display(new_species)  # if running in IPython --pylab mode, draws the picture!
 
                 # Add new species
-                reactions_moved_from_edge = self.add_species_to_core(new_species)
+                if new_species not in self.core.species:
+                    reactions_moved_from_edge = self.add_species_to_core(new_species)
+                else:
+                    reactions_moved_from_edge = []
 
             elif isinstance(new_object, tuple) and isinstance(new_object[0], PDepNetwork) and self.pressure_dependence:
                 pdep_network, new_species = new_object