Implement algorithm for special periodic and a periodic systems

[PicoCentauri]

So far we have fast implementations for 3D periodic systems as well as $N^2$ scaling calculator for real space systems. We might want to consider including additional periodic cases and probably more important a faster real space algorithm that is better suited for large systems with free boundary conditions.

A couple of possible extensions:

• 1D periodic systems: https://doi.org/10.1063/1.2052647
• (real) 2D periodic systems: https://www.sciencedirect.com/science/article/pii/S0010465502005866
• (approximated) 2D periodic system: Using 3D Ewald plus a correction term: http://scitation.aip.org/content/aip/journal/jcp/131/9/10.1063/1.3216473
• free boundary system: https://doi.org/10.1063/1.2746328

Regarding the first two there is also a shorter introduction in the Appendix of the espresso docs.

[tulga-rdn]

I'll be on it