Generalize ReagentUnit into FixedPoint2 and use it for damage calculations (#5151)

* Damage units

* sum ext method

Content.Shared/MobState/State/IMobState.cs

@@ -1,4 +1,5 @@
 using Content.Shared.ActionBlocker;
+using Content.Shared.FixedPoint;
 using Robust.Shared.GameObjects;
 
 namespace Content.Shared.MobState.State
@@ -36,6 +37,6 @@ namespace Content.Shared.MobState.State
         /// <summary>
         ///     Called when this state is updated.
         /// </summary>
-        void UpdateState(IEntity entity, int threshold);
+        void UpdateState(IEntity entity, FixedPoint2 threshold);
     }
 }