Make basis choices in irrfbz_path and wignerseitz agree

[thchr]

Right now, wignerseitz will iterate to give points in a cartesian basis (but embeds the underlying basis) but irrfbz_path iterates to return points in the lattice basis (and doesn't embed the underlying basis - but can be converted via cartesianize!).

We should make these two consistent. Since the core utility for other applications will be irrfbz_path, and since they - I guess - would be more likely to want elements in a lattice basis, I guess it makes more sense to have everything in the lattice basis by default, and make it more ergonomic to convert to a cartesian basis (by referencing an embedded basis).

The downside to embedding the basis e.g. in a KPath is that we then need to be able to go between conventional and primitive basis vectors - which means either depending on Crystalline or vendoring parts of it.