OpenMPI IO garbage also in tools' output

[ykempf]

In some cases, e.g. vlsvextract produces garbage VLSV files if the same trick as in vlasiator proper is not applied.
https://github.com/fmihpc/vlasiator/blob/dev/vlasiator.cpp#L259
That's the spot for reference, we should plonk this into the compiled tools as well.