openPMD: Directory Cleanup, 6 Digits (#1149)

This reorders the naming of diagnostics to be a bit more readable.

Currently it was:
```bash
$ ls Bin/diags/
diag1bp/<N>.bp
diag1h5/<N>.h5
...
diag100000
diag100100
     <N>
     ...
```

Now it proposes for openPMD output
```bash
$ ls Bin/diags/
diag1/openpmd_<N>.bp
diag1/openpmd_<N>.h5
```

This is also a bit easier for some tooling (i.e. yt and
openPMD-viewer that expect full consistent dirs atm.) and keeps
things nicely together.

Also bumps the zero-padding to use consistently 6 digits for
iterations in output files (<1M iterations), since sims (ions)
with >100k steps are not uncommon.

This does not yet change plot files, since this is still requiring
to make the regression testing scripts more flexible.

0 files changed, 0 insertions, 0 deletions