Refactor to encapsulate strain logic into Skill class

As strains are an implementation detail of the current Skill calculations, it makes sense that strain related logic should be encapsulated within the Skill class.

osu.Game.Rulesets.Catch/Difficulty/Skills/Movement.cs

@@ -20,6 +20,8 @@ namespace osu.Game.Rulesets.Catch.Difficulty.Skills
 
         protected override double DecayWeight => 0.94;
 
+        protected override int SectionLength => 750;
+
         protected readonly float HalfCatcherWidth;
 
         private float? lastPlayerPosition;