new_contents: class Sim(object):
      def __init__(mesh, Ms):
          self.mesh = mesh
          self.Ms = Ms
          self.energies = []
  
      def add_exchange(self, A):
          self.energies.append(('Exchange', A))
  
      def add_demag(self):
          self.energies.append(('Demag', None))
  
      def add_zeeman(self, H):
          self.energies.append(('Zeeman', H))
  
      def set_m(self, m_init):
          self.m_init = m_init
  
      def create_mif(self):
          pass
  
      def run_until(self, t_sim):
          pass
  
  
subject: Add simulation class.