Add cell list neighbour search for non-bonded interactions

[mlund]

Currently the only way to skip interactions is to use a group-to-group based cutoff for molecular groups. This is inefficient for small molecules, of if atomic groups are in use. The idea is to implement a cell list where particles and particle mass centra are assigned to a grid point. A basic structure for this already exists.

Things to consider:

• who holds the cell list (space, nonbonded?)
• periodic boundaries or not
• updating (particle movement, volume scaling)

The Aboria library is a particle container, i.e. corresponding to our std::vector<Particle> and implements for neighbour list routines. It requires boost, though.