Band structure ok

Merge request for the band-structure feature, only with the non-controversial commits. A test is also added to cover the new feature.

Add the possibility to define a k-point path, with the variable KPointsPath (see documentation). The kpoint module has been cleaned to have a better and safer control of the different k-point variables.

If KPointsPath is used for an unocc calculation, the code will write a bandstructure file that can be straightforwardly plotted, for instance using gnuplot command: plot for [col=5:5+nst] 'static/bandstructure' u 1:col w l t 'Band '.(col-4)

Note that in this case, the no restart information is written.