added expandable list of ingredients

lib/models/ingredient.dart

@@ -1,6 +1,8 @@
+import 'unit.dart';
+
 class Ingredient {
   final String title;
-  List<String> possibleUnits = [];
+  List<Unit> possibleUnits = [];
   List<String> preferredBrands = [];
 
   Ingredient({

lib/models/unit.dart

@@ -0,0 +1,17 @@
+class Unit {
+  final String name;
+  final UnitType type;
+  final System system;
+
+  Unit(this.name, this.type, {this.system = System.metric});
+}
+
+enum System { metric, imperial, neutral }
+
+enum UnitType {
+  metric,
+  imperial,
+  fluid,
+  weight,
+  count,
+}