spatially resolved observables, e.g., density profiles, velocity field
The idea is to provide a class of modules "binned_particle_group". These modules would generate a map between a tesselation of space to the particles in a particle_group. Such tesselations may be derived from certain curvilinear coordinates, e.g., Cartesian, cylindrical, spherical coordinates with either equi-sized bins or arbitrary grids. The first step is of course to implement binning into regular grids along Cartesian axes, analogous to the construction of the Verlet cell lists.
The binned_particle_group is then passed instead of particle_group to (new) observable modules supporting fields. This will allow the evaluation of density, temperature, or velocity fields or even stress tensor fields (which require special care and include the recomputation of the interactions).