Physical time in IO is incorrect

[SeverinDiederichs]

In the new pipeline, the physical time in the IO files is not yet set correctly, because the other ranks are not aware of the dt of the other ranks. The dt needs to be communicated, so each rank knows its absolute time, which it should write to file.

This is also a good opportunity for cleaning:
Previously, the beam and the field data were not at the same time, when written to file.

The correct approach would be to: write the beam to file, calculate the fields on the box (which simultaneously pushes the beam to t += dt), and then write the field to file.