new_contents: from unittest import TestCase
  from atomium.structures import Model, Atom
  
  class ModelBuildingTests(TestCase):
  
      def test_can_build_model(self):
          model = Model()
          self.assertEqual(model.atoms(), set())
          self.assertEqual(model.mass(), 0)
  
          atom1 = Atom("N", 12.0, 11.5, 1.5)
          atom2 = Atom("C", 12.5, 10, 2)
  
          model.add_atom(atom1)
          model.add_atom(atom2)
  
          self.assertEqual(model.atoms(), set([atom1, atom2]))
          self.assertAlmostEqual(model.mass(), 26, delta=0.1)
  
  
  
  class AtomInteractionTests(TestCase):
  
      def test_can_get_atom_relations(self):
          atom1 = Atom("N", 12.0, 11.5, 1.5)
          atom2 = Atom("C", 12.5, 10, 2)
  
          self.assertAlmostEqual(atom1.distance_to(atom2), 1.658312, delta=0.0005)
  
subject: Add Model itests