Center of pulsetunnel is now between 2 modules

The LED strip has an even number of modules, so this change makes the
animation completely symmetrical.

pulsetunnel.lua

@@ -27,7 +27,7 @@ function periodic()
   local redEnergy = get_energy_in_band(0, 400);
   local greenEnergy = get_energy_in_band(400, 4000);
   local blueEnergy = get_energy_in_band(4000, 22000);
-  local centerIndex = 2 * center_module;
+  local centerIndex = 2 * center_module + 1;
 
   maxRedEnergy = maxRedEnergy * COOLDOWN_FACTOR
   if redEnergy > maxRedEnergy then