np.NaN deprecated for np.nan

Running the tutorial with numpy v2.0 yields an error due to `np.NaN`. Both `np.NaN` and `np.nan` have been used, changed the remaining two NaN to nan.

pyMBE.py

@@ -714,14 +714,14 @@ def copy_df_entry(self, name, column_name, number_of_copies):
             else:
                 df_by_name = df_by_name[df_by_name.index == df_by_name.index.min()] 
                 df_by_name_repeated = pd.concat ([df_by_name]*(number_of_copies), ignore_index=True)
-                df_by_name_repeated[column_name] =np.NaN
+                df_by_name_repeated[column_name] =np.nan
             # Concatenate the new particle rows to  `df`
             self.df = pd.concat ([self.df,df_by_name_repeated], ignore_index=True)
         else:
             if not df_by_name[column_name].isnull().values.any():     
                 df_by_name = df_by_name[df_by_name.index == df_by_name.index.min()] 
                 df_by_name_repeated = pd.concat ([df_by_name]*(number_of_copies), ignore_index=True)
-                df_by_name_repeated[column_name] =np.NaN
+                df_by_name_repeated[column_name] =np.nan
                 self.df = pd.concat ([self.df,df_by_name_repeated], ignore_index=True)
         return