The MField database stores the dark matter density fluctuation field smoothed with various
smoothing radii (in the MField table) and information about the snapshots of the simulation.
The goal of the MField table is to give a simplistic way to find out information about the environment
of an object. To this end each grid cell is identified by its Penao-Hilbert key. This same key has been assigned to
all objects (halos and galaxies), in that case identifying the cell they reside in. For a given object the
environment is now a simple lookup of the corresponding cell in the density field table.