Digits in LogMFD tests

[GiovanniBussi]

This is for: @tetsuyamorishita

I recently parallelized simplemd with OpenMP. Running with two threads (which we do by default on GitHub Actions) slightly changes the trajectories. The LogMFD tests were failing since they write too many digits, so I forced them to single thread in 3231968.

I would suggest changing the number of digits written in the LogMFD files to avoid potential numerical problems with future changes.

Thanks!