[max-models]

Adds Field.deposit_density(positions, values, weighting=LINEAR): it scatters the marker values into a zero copy of the field, exchanges the halos on a split field, divides by the node volumes and adds the result. With the products w, w v, w v v, ... as components it gives the velocity moments of any order (n, n u, n, ...) in one deposit; the products come from mpiparticles (moment_values), so mpifield stays free of velocities.

• Field.node_volumes(): the volume each node collects, half a cell on a bounded edge.
• Field.on_grid takes num_ghost_points, so a split field can carry the halo a quadratic deposit needs.
• A method card with a SymPy rule (the density times the volumes is the total).
• Tests: conservation for both weightings and all edge types, uniform density up to the edges, the split deposit against the serial one under MPI.
• Example 14 derives density, flow, temperature and heat flux from the moments of a drifting Maxwellian.