Trivial cluster
cluster_single(a, b = a, c = b, phi = 0, theta = 0, psi = 0)
list of class cluster with fields: positions, sizes, angles
A single particle cluster
Other user_level cluster: cluster_array,
  cluster_ball, cluster_chain,
  cluster_dimer, cluster_helix,
  cluster_shell
cl = cluster_single(10)