convert custom activity rate from list to numpy array of type 'double' to satisfy cython input

src/seismic_hazard_forecasting.py

@@ -320,8 +320,8 @@ verbose: {verbose}")
     lambdas = [None]
     if custom_rate != None and forecast_select:
         logger.info(f"Using activity rate specified by user: {custom_rate} per {time_unit}")
-        lambdas = [custom_rate]
-        lambdas_perc = [1]
+        lambdas = np.array([custom_rate], dtype='d')
+        lambdas_perc = np.array([1], dtype='d')
 
     elif rate_select:
         logger.info(f"Activity rate modeling selected")