class_docstr: The Lennard Jones potential
  
  a mathematically simple model that approximates the interaction between a
  pair of neutral atoms or molecules.
  https://en.wikipedia.org/wiki/Lennard-Jones_potential
  
  E = sum_ij V(r_ij)
  
  where r_ij is the cartesian distance between atom i and atom j, and the
  pair potential has the form
  
  V(r) = 4 * eps * ( (sigma / r)**12 - (sigma / r)**6
  
  Notes
  -----
  the double loop over many atoms makes this *very* slow in Python.  If it
  were in a compiled language it would be much faster.
code: def __init__(self, eps=1.0, sig=1.0):
      self.sig = sig
      self.eps = eps
  
imports: import time
  import numpy
  from numpy import sin, cos, pi, exp, sqrt, abs
  from scipy.optimize import rosen
label_desc: Use image node_id 1 to create a new LJ object  with example: obj = LJ(eps, sig)