Changing results with changing number of procs or threads

Hello,

With my test case for gas combustion calculation, I remark that I obtain different results depending of the number of processors.
I attached a small test case with 1, 2 and 4 procs and with a studymanager to run the cases. If you compare the curves in report_detailed.pdf (result of the studymanager) you will see that the curves are different for each case. This case is limited at 50s but if you push at 300s, the difference become bigger.

Regards,

Yohann
[Test-procs.zip](https://github.com/code-saturne/code_saturne/files/5674380/Test-procs.zip)
[report_detailed.pdf](https://github.com/code-saturne/code_saturne/files/5674383/report_detailed.pdf)

Start by unpacking and running the attached Test-procs.zip cases through the studymanager with 1, 2, and 4 processors. Compare the resulting curves in report_detailed.pdf, including the longer 300-second run; done means identifying and correcting the cause of the processor-dependent results or documenting the confirmed limitation.

Written by the indexing model from the issue text.