Add Coulomb Force Field and Long Range Solver

[crtrott]

This is a longer term goal. We need to add both the short range coulomb interaction as well as a long range interaction.

[junghans]

Should we use (ScaFaCos)[http://www.scafacos.de/] as a start?

Their interface seem easy to use: https://github.com/scafacos/scafacos/blob/master/test/c/test_p3m.c#L193
and it would give us 5 Coulomb methods to compare with.