Adds device powercells

Adds a type of smaller power cell for powering devices. Not used by much
at the moment, but it's one step closer to a more coherent picture of
power cells.

code/modules/mob/living/silicon/robot/robot.dm

@@ -594,6 +594,8 @@
 			user << "Close the panel first."
 		else if(cell)
 			user << "There is a power cell already installed."
+		else if(W.w_class != 3)
+			user << "\The [W] is too [W.w_class < 3? "small" : "large"] to fit here."
 		else
 			user.drop_item()
 			W.loc = src