Billing: Add EditBillingData class

Elwig/Helpers/Billing/BillingData.cs

@@ -62,7 +62,7 @@ namespace Elwig.Helpers.Billing {
             Mode = (mode == "elwig") ? CalculationMode.Elwig : CalculationMode.WgMaster;
         }
 
-        public static JsonObject ParseJson(string json) {
+        protected static JsonObject ParseJson(string json) {
             if (Schema == null) throw new InvalidOperationException("Schema has to be initialized first");
             try {
                 var errors = Schema.Validate(json);
@@ -117,7 +117,7 @@ namespace Elwig.Helpers.Billing {
             return dict;
         }
 
-        public Dictionary<int, Curve> GetCurves() {
+        protected Dictionary<int, Curve> GetCurves() {
             var dict = new Dictionary<int, Curve>();
             var curves = GetCurvesEntry();
             foreach (var c in curves) {