created different models

lib/models/unit.dart

@@ -0,0 +1,9 @@
+import '../src/enums.dart' show UnitType, System;
+
+class Unit {
+  final String name;
+  final UnitType type;
+  final System system;
+
+  Unit(this.name, this.type, {this.system = System.metric});
+}