[WIP] Additional exx kernels: Voxel averaged and Wigner-Seitz truncation

[toschaefer]

Two additional useful strategies for the Coulomb kernel for electron repulsion integrals (e.g. for exx).

Wigner-Seitz truncation
R. Sundararaman, T. A. Arias. Phys. Rev. B 87, 165122 (2013)

Voxel averaged (mean potential)
T. Schaefer et al, J. Chem. Phys. 160, 051101 (2024)
This requires Gauss-Legendre quadrature points, hence I added the package FastGaussQuadrature to the dependencies.

[antoine-levitt]

Nice! Particularly interested in the Wigner-Seitz truncation - is there any reason to do anything else? It seems to me this is the superior approach? Do we really want to support the voxel averaged? Also maybe some opportunities to share code with ewald.jl, but I haven't looked too closely.

It'd be nice to have a generalized API for all these functions that return centered functions (we have a couple of those: densities guesses, local pseudos, projectors, coulomb kernels, etc.). I think long-term we want the ability to get their real-space and partial Fourier transforms (wrt 1 or 2 dimensions), so we can do eg 2D materials. But maybe it's not worth unifying since they have quite different requirements (numerical psp vs analytic but singular coulomb really are quite different computationally)

CC @denizunel

[mfherbst]

Also maybe some opportunities to share code with ewald.jl,

I think so, but I'd keep that for later, same as the generalized API. I'm currently looking at this on the side to streamline the code a bit, will hopefully push a review in the next week.