minor changes to grid walking

grid/Grid.h

@@ -213,6 +213,14 @@ public:
 		return GridNodeBBox(node, gridSize_cm);
 	}
 
+	/** is this node part of a non-plain stair/escalator */
+	bool isPlain(const T& n1) const {
+		for (const T& n2 : neighbors(n1)) {
+			if (n2.z_cm != n1.z_cm) {return false;}
+		}
+		return true;
+	}
+
 	/**
 	 * get an UID for the given point.
 	 * this works only for aligned points.
@@ -469,11 +477,11 @@ public:
 
 
 
-	NeighborForEach neighbors(const int idx) {
+	NeighborForEach neighbors(const int idx) const {
 		return neighbors(nodes[idx]);
 	}
 
-	NeighborForEach neighbors(const T& node) {
+	NeighborForEach neighbors(const T& node) const {
 		return NeighborForEach(*this, node._idx);
 	}