missing k-point for discontinuous paths

[giovannipizzi]

When a discontinuous band-structure path is asked for, e.g.

G 0 0 0 X 0.5 0 0 
L 0.5 0.5 0.5 G 0 0 0

The last point (X in this case) is not printed. E.g. if the G-X line 50 points are considered, only points from 1 to 49 are printed.
I think the idea behind this is that, if the path is continuous, the X point will be the first of the next path, so it is not needed to compute it.

However, for discontinuous paths, it is needed to compute it. In the example above, otherwise, the eigenvalues at X are never computed.

I propose to compute also the last point if we detect that the path is discontinuous (I would do a check of both the coordinates and the label, if at least one of these is different, I consider this discontinuous).

[giovannipizzi]

• Check that paths from the same point to the same point work
• Check that the output of gnu plot and xmgrace is correct
• Check that the path length is correct
• Add documentation for the new file with the labels and corresponding indices
• Add test

[giovannipizzi]

Note that in the current version (Wannier 1.x and 2.x), the length of the path for the first point in a new segment can be slightly off. I will fix also this.