Fix the M/h computation in the CCZ4/Writers/TimingStatisticsWriter.cpph
From a mail at Luke:
I just noticed there is something wrong in the M/h determination:
As explained in my last mail to Tobias and you, I just divide these two numbers. However, the time in the first column of stdout measures the time since program start.
In ExaHyPE, the grid setup sometimes takes a considerable amount -- like 10 minutes. If you measure the M/h straight after the first timesteps after these 10 minutes, you get of course totally wrong numbers. However, if you measure after 1000 minutes runtime, the 10 minutes grid setup do not change the result so much.
It is not hard to substract the time the grid setup needs in order to improve the correctness of the number. You can do this either by hand (just look up when the first time step started) or we can do this in code (CCZ4/Writers/TimingStatisticsWriter.cpph).