A ball of particles on a cubic lattice
cluster_ball(N, R0 = 15, a = 1, b = 1, c = b)
list of class cluster with fields: positions, sizes, angles
Identical particles fill a sphere with a cubic lattice
Other user_level cluster: cluster_array,
  cluster_chain, cluster_dimer,
  cluster_helix, cluster_shell,
  cluster_single
b = cluster_ball(100)#>