Export: Refactor interfaces and classes

Elwig/Helpers/Export/Elba.cs

@@ -1,14 +1,14 @@
-using Elwig.Models;
+using Elwig.Helpers.Billing;
 using System;
-using System.Collections.Generic;
-using System.Threading.Tasks;
 
 namespace Elwig.Helpers.Export {
-    public class Elba : IBankingProvider {
+    public class Elba : Csv<Transaction>, IBankingExporter {
 
-        public string FileExtension => "elba";
+        public static new string FileExtension => "elba";
 
-        public Task Export(string filename, int avnr, IEnumerable<Member> members) {
+        public Elba(string filename) : base(filename) { }
+
+        public override string FormatRow(Transaction row) {
             throw new NotImplementedException();
         }
     }