Implementation of SPME

[GardevoirX]

The SPME uses a different interpolation scheme from PME, and thus does not have the PME's discontinuity problem in forces.

This is the main reference. The only thing we need to implement is the kernel (the interpolation scheme is already implemented), which reads

$$\hat{G}\left(\mathbf{k}\right) = B\left(\mathbf{k}\right) \tilde{g}\left(\mathbf{k}' \right) \tilde{\gamma} \left(\mathbf{k} ' \right)$$

where $\tilde{g}$ and $\tilde{\gamma}$ are what we already have, so the things left to do are $B$ and $\mathbf{k}'$.