Makes Airtight ECS. (#4351)

* Makes Airtight ECS.

* Remove atmos holdovers while at it!

Content.Server/Atmos/IGasMixtureHolder.cs

@@ -0,0 +1,25 @@
+using Content.Server.Atmos.EntitySystems;
+using Robust.Shared.GameObjects;
+
+namespace Content.Server.Atmos
+{
+    public interface IGasMixtureHolder
+    {
+        public GasMixture Air { get; set; }
+
+        public virtual void AssumeAir(GasMixture giver)
+        {
+            EntitySystem.Get<AtmosphereSystem>().Merge(Air, giver);
+        }
+
+        public GasMixture RemoveAir(float amount)
+        {
+            return Air.Remove(amount);
+        }
+
+        public GasMixture RemoveAirVolume(float ratio)
+        {
+            return Air.RemoveRatio(ratio);
+        }
+    }
+}