Canister fixes and rework (#4266)

* Kills PipeNode ConnectToContainedEntities and EnvironmentalAir with fire.

* Reaction IGasMixtureHolder is nullable.

* Reworks canisters

* Adds PortablePipeNode and PortPipeNode, which connect to each other.

Content.Server/NodeContainer/Nodes/PortPipeNode.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+using Robust.Shared.Serialization.Manager.Attributes;
+
+namespace Content.Server.NodeContainer.Nodes
+{
+    [DataDefinition]
+    public class PortPipeNode : PipeNode
+    {
+        public override IEnumerable<Node> GetReachableNodes()
+        {
+            foreach (var node in PipesInTile())
+            {
+                if (node is PortablePipeNode)
+                    yield return node;
+            }
+
+            foreach (var node in base.GetReachableNodes())
+            {
+                yield return node;
+            }
+        }
+    }
+}