better handle of empty config and changing length of string

src/pypromice/process/L2toL3.py

@@ -609,9 +609,13 @@ def thermistorDepth(df_in, site, station_config):
     logger.info('Calculating thermistor depth')
 
     # Convert maintenance_info to DataFrame for easier manipulation
-    maintenance_string = pd.DataFrame(station_config.get("string_maintenance",[]))
+    maintenance_string = pd.DataFrame(
+        station_config.get("string_maintenance",[]),
+        columns = ['date', 'installation_depths']
+        )
     maintenance_string["date"] = pd.to_datetime(maintenance_string["date"])
     maintenance_string = maintenance_string.sort_values(by='date', ascending=True)
+
     temp_cols_name = ['t_i_'+str(i) for i in range(12) if 't_i_'+str(i) in df_in.columns]
     num_therm = len(temp_cols_name)
     depth_cols_name = ['d_t_i_'+str(i) for i in range(1,num_therm+1)]
@@ -647,7 +651,8 @@ def thermistorDepth(df_in, site, station_config):
         new_depth = maintenance_string.loc[
                                         maintenance_string.date == date
                                     ].installation_depths.values[0]
-        for i, col in enumerate(depth_cols_name):
+
+        for i, col in enumerate(depth_cols_name[:len(new_depth)]):
             tmp = df_in[col].copy()
             tmp.loc[date:] = (
                 new_depth[i]