input: Write a python function to get all descendants of a given node in a family tree.
  assert isinstance(family.get_all_descendants(node1), list)
  assert all(isinstance(descendant, Node) for descendant in family.get_all_descendants(node1))
  assert node2 in family.get_all_descendants(node1)
  assert node3 in family.get_all_descendants(node1)
output: def get_all_descendants(self, node):
      descendants = []
  
      def find_descendants(n):
          for child in n.children:
              descendants.append(child)
              find_descendants(child)
      
      find_descendants(node)
      return descendants