old_contents: import mesh
  import sim
  
  
  def test_sim_basic_square():
      testmesh = mesh.Mesh((10, 20, 30), (3, 3, 3), scale=1e-9)
      sim.Sim(testmesh, 1e4)
  
new_contents: import mesh
  import sim
  from energies.demag import Demag
  
  def test_sim_basic_square():
      testmesh = mesh.Mesh((10, 20, 30), (3, 3, 3), scale=1e-9)
      sim.Sim(testmesh, 1e4)
      sim.add_energy(Demag())
  
subject: Cover add_energy fn in test_sim.py