Created basic Timer class and TimerEntity

Signed-off-by: Andreas Fahrecker <AndreasFahrecker@gmail.com>

lib/models/timer.dart

@@ -0,0 +1,33 @@
+import 'package:meta/meta.dart';
+import 'package:time_progress_calculator/persistence/timer_entity.dart';
+
+@immutable
+class Timer {
+  final DateTime startTime;
+  final DateTime endTime;
+
+  Timer(this.startTime, this.endTime);
+
+  Timer copyWith({DateTime startTime, DateTime endTime}) {
+    return Timer(startTime ?? this.startTime, endTime ?? this.endTime);
+  }
+
+  @override
+  int get hashCode => startTime.hashCode ^ endTime.hashCode;
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is Timer &&
+          runtimeType == other.runtimeType &&
+          startTime == other.startTime &&
+          endTime == other.endTime;
+
+  TimerEntity toEntity() {
+    return TimerEntity(startTime, endTime);
+  }
+
+  static Timer fromEntity(TimerEntity entity) {
+    return Timer(entity.startTime, entity.endTime);
+  }
+}