added comments and refactored code

lib/model/flood_station.dart

@@ -30,12 +30,15 @@ class FloodStation {
         riverName: parseStringValue(json['riverName']),
       );
 
+  // sometimes the API returns a String instead of a double
   static double parseDoubleValue(dynamic value) {
     if (value is double) return value;
     if (value is String) return double.parse(value);
     return 0;
   }
 
+  // sometimes the API returns a list of labels that are basically identical
+  /// if [value] is a List, the method return the first item
   static String parseStringValue(dynamic value) {
     if (value is String) return value;
     if (value is List<dynamic>) return value[0];