so i made a slag heater (#6925)

* Add slag fueled heater

* i have no idea why i have to do this but i do

Co-authored-by: Blue <415bluewolf@gmail.com>
Co-authored-by: Anuken <arnukren@gmail.com>

core/src/mindustry/content/ErekirTechTree.java

@@ -197,6 +197,10 @@ public class ErekirTechTree{
                                         });
                                     });
 
+                                    node(slagHeater, () -> {
+
+                                    });
+                                    
                                     node(atmosphericConcentrator, Seq.with(new OnSector(four)), () -> {
                                         node(cyanogenSynthesizer, Seq.with(new OnSector(four)), () -> {