[PyBigDFT] Fix issue parsing geopt log files.

Commit 38662952 introduced these two lines in Logfiles.py: datadir = os.path.join(self.log['outdir'], self.log['Data Writing directory'])

But it has no way to work in a geometry optimisation. The header is not reprinted at each step of the geometry optimisation, thus outdir is not a key of the log dictionary after the first one. So the call line 431 in failing for step 2…

In a word, PyBigDFT is broken for geometry runs :/ But it is now fixed and working again \o/

I profit of the MR in PyBigDFT to propose a dump() function in BrillouinZone, line in DoS, to be able to visualise the band structure in Gnuplot for instance.