engine: Layers(...) was actually checking bounds based on X dimension…

… instead of Z

engine/shape.go

@@ -98,7 +98,7 @@ func ZRange(a, b float64) Shape {
 
 // Cell layers #a (inclusive) up to #b (exclusive).
 func Layers(a, b int) Shape {
-	Nz := Mesh().Size()[0]
+	Nz := Mesh().Size()[Z]
 	if a < 0 || a > Nz || b < 0 || b < a {
 		util.Fatal("layers ", a, ":", b, " out of bounds (0 - ", Nz, ")")
 	}