Add difficulty calculation

Adds base classes for difficulty calculations, hooks them up with
carousel container, and adds a port of the osu difficulty calculator.

osu.Game.Modes.Catch/CatchDifficultyCalculator.cs

@@ -0,0 +1,27 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Game.Beatmaps;
+using osu.Game.Modes.Catch.Objects;
+using osu.Game.Modes.Objects;
+using System;
+using System.Collections.Generic;
+
+namespace osu.Game.Modes.Catch
+{
+    public class CatchDifficultyCalculator : DifficultyCalculator<CatchBaseHit>
+    {
+        protected override PlayMode PlayMode => PlayMode.Catch;
+
+        public CatchDifficultyCalculator(Beatmap beatmap) : base(beatmap)
+        {
+        }
+
+        protected override HitObjectConverter<CatchBaseHit> Converter => new CatchConverter();
+
+        protected override double ComputeDifficulty(Dictionary<String, String> categoryDifficulty)
+        {
+            return 0;
+        }
+    }
+}