code: def _get_deltas(self, sites):
          
          siteclass = sites.siteclass
          delta_C = np.zeros_like(siteclass, dtype=np.float)
          delta_C[siteclass == b'C'] = 1
  
          delta_D = np.zeros_like(siteclass, dtype=np.float)
          delta_D[siteclass == b'D'] = 1
  
          return delta_C, delta_D
docs: Return delta's for equation 4
          delta_C = 1 for site class C, 0 otherwise
          delta_D = 1 for site class D, 0 otherwise