Re-add support for the Mueller kpoint scheme

[Andrew-S-Rosen]

In older versions of Pymatgen, there was a pymatgen.ext.jhu utility that would allow for generation of kpoints using the scheme from Mueller but it required access to a server (which is maybe why it was removed?).

As shown here https://gitlab.com/muellergroup/k-pointGridGenerator there is a way to generate these kpoints without the server (but with an optional dependency). It would be a nice feature to add one day. Maybe if I discover some free time.......... logging the idea here anyway.

[shyuep]

Yup it was removed because the server was far too fragile. Would be happy to have it back as an optional dependency.