Made GradeAverage persistent

lib/storage/gradeAverages/gradeAveragesSettings.dart

@@ -0,0 +1,15 @@
+
+import 'package:json_annotation/json_annotation.dart';
+
+part 'gradeAveragesSettings.g.dart';
+
+@JsonSerializable()
+class GradeAveragesSettings {
+  bool useGradeSystem;
+  List<int> inputs;
+
+  GradeAveragesSettings({required this.useGradeSystem, required this.inputs});
+
+  factory GradeAveragesSettings.fromJson(Map<String, dynamic> json) => _$GradeAveragesSettingsFromJson(json);
+  Map<String, dynamic> toJson() => _$GradeAveragesSettingsToJson(this);
+}