Kpoints writer refactoring for VASP

Checklist

• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• New features and API Changes are described in doc/releasenotes.rst.

Summary

This is a first and very small step to progress with issue #1340. It turned out most work on the kpoints front was already done last March by @askhl . Further back, there was a different approach with a new kpoints writer by @martin-schlipf (!2588) and later I tried to extend this using some of Adam Jacksons work in !2865.

While I think it would be nice to also be able to write in VASPs line mode for bandstructures or generalized regular meshes, it is much more likely that the ASE-internal machinery for BandPaths is used. (I am not sure if there are generalized meshes available, but then, also in that case, a list of kpoints with weigths can be passed to the current writer).

Further work might be necessary after review and I am happy to work on that.