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/Ruleset.cs

@@ -32,6 +32,8 @@ namespace osu.Game.Modes
 
         public abstract HitObjectParser CreateHitObjectParser();
 
+        public abstract DifficultyCalculator CreateDifficultyCalculator(Beatmap beatmap);
+
         public static void Register(Ruleset ruleset) => availableRulesets.TryAdd(ruleset.PlayMode, ruleset.GetType());
 
         protected abstract PlayMode PlayMode { get; }