ENH/TST: Vasp line-mode reading

VASP provides a syntax to specify k-point paths in "LINE-MODE" as a series of special points and fixed number of points spaced between them. While this is generally inferior to sampling evenly in k-space, it's a convenient syntax and users may reasonably wish to read in their existing band structure calculations with ASE.

Closes #352

This MR

• Provides support for line-mode in the read_kpoints method of the GenerateVaspInput class shared by all Vasp calculators
• Removes a redundant block from the Vasp XML reader which was getting stuck on line-mode calculations
• Adds tests for the read_kpoints method over all supported KPOINTS file formats

While implementing this I noted a couple of other deficiencies in read_kpoints that I do not propose to address as part of this MR:

• Nothing is done with the grid offset information. If a user reads a KPOINTS file with offsets and then uses that to start a new calculation, they will get a different k-point grid. There is also no facility in GenerateVaspInput to write such information; the offsets are hard-coded to 0 0 0.
• When reading an explicit list of k-points, it is assumed that all remaining lines are k-point positions. This is not a safe assumption, as calculations with tetrahedron integration would provide an explicit list of tetrahedra following the k-point list. The correct way to read the k-point list (even if we do disregard the tetrahedra) is to use the number in the second line of the file to identify the number of k-points that should be read.