ENH: Vasp calculator restart with explicit kpts

The Vasp calculator already has support for writing explicit k-points in cartesian or reciprocal coordinates by setting kpts to an (m x 3) array (with equal weighting) or an (m x 4) array (with weights in fourth column).

This merge request adds support to the read_kpoints() method so that calculations can be restarted or imported from an existing calculation folder.

This merge request does not include the bug fix to the same method in !266 (merged) ; no difficulties are anticipated in merging these together.