modify reading pandas way

mrdna/readers/segmentmodel_from_oxdna.py

@@ -114,7 +114,7 @@ def mrdna_model_from_oxdna(coordinate_file, topology_file,virt2nuc=None,get_nt_p
         for i in nt_prop.index:
             if nt_prop.loc[i]["threeprime"] in nt_prop.index:
                 if nt_prop.loc[nt_prop.loc[i]["threeprime"]]["vh"]!=nt_prop.loc[i]["vh"]:   
-                    nt_prop["stack"][i]=-1
+                    nt_prop.loc[i]["stack"]=-1
         
         bp=-np.ones(len(nt_prop.index),dtype=int)
         counter=0