Data-oriented Construction System (#2152)

- Powerful
- Data-oriented
- Approved by PJB
- Powered by node graphs and AI pathfinding
- Coded by the same nerd who brought you atmos

Co-authored-by: Exp <theexp111@gmail.com>

Content.Shared/Construction/NestedConstructionGraphStep.cs

@@ -0,0 +1,41 @@
+using System.Collections.Generic;
+using System.IO;
+using Robust.Shared.Serialization;
+using Robust.Shared.Utility;
+using YamlDotNet.RepresentationModel;
+
+namespace Content.Shared.Construction
+{
+    public class NestedConstructionGraphStep : ConstructionGraphStep
+    {
+        public List<List<ConstructionGraphStep>> Steps { get; private set; } = new List<List<ConstructionGraphStep>>();
+
+        public void LoadFrom(YamlMappingNode mapping)
+        {
+            if (!mapping.TryGetNode("steps", out YamlSequenceNode steps)) return;
+
+            foreach (var node in steps)
+            {
+                var sequence = (YamlSequenceNode) node;
+                var list = new List<ConstructionGraphStep>();
+
+                foreach (var innerNode in sequence)
+                {
+                    var stepNode = (YamlMappingNode) innerNode;
+                    var step = ConstructionGraphEdge.LoadStep(stepNode);
+
+                    if(step is NestedConstructionGraphStep)
+                        throw new InvalidDataException("Can't have nested construction steps inside nested construction steps!");
+
+                    list.Add(step);
+                }
+
+                Steps.Add(list);
+            }
+        }
+
+        public override void DoExamine(FormattedMessage message, bool inDetailsRange)
+        {
+        }
+    }
+}