Fix grackle cooling statistics plotting scripts

Some grackle cooling plotting scripts weren't up to date. In particular, they were looking for the energy.txt file instead of the statistics.txt file. This fixes that.

examples/Cooling/CoolingBox/coolingBox.yml

@@ -23,6 +23,9 @@ Snapshots:
 Statistics:
   delta_time:          1e-3 # Time between statistics output
 
+Scheduler:
+  tasks_per_cell: 64
+
 # Parameters for the hydrodynamics scheme
 SPH:
   resolution_eta:        1.2348   # Target smoothing length in units of the mean inter-particle separation (1.2348 == 48Ngbs with the cubic spline kernel).