small updates on seissolxdmf and seissolxdmfwriter

[Thomas-Ulrich]

• fix format of extracted seissol output (
  <Grid Name="TimeSeries" GridType="Collection" CollectionType="Temporal">
  should appear only once and not at each time step. Paraview was not complaining about that, but SeisSol/Sampler does.
• print available variables when the variable to read is not available.
• replace corrupted time-steps (typically the last one when seissol is interrupted writing) with nans instead of 0s.
• change setup.py to pyproject.toml (more recent format).

[palgunadi1993]

hi Thomas,
I also want to add bug on this line:
https://github.com/SeisSol/Visualization/blob/master/seissolxdmf/seissolxdmf/seissolxdmf.py#L285

should it be self.ReadDataChunk()?

Thanks.
Kadek