Add a reference for CRAIG

[amontoison]

@article{craig-1955,
  title        = {{The N-step iteration procedures}},
  author       = {J. E. Craig},
  year         = 1955,
  journal      = jmphy,
  volume       = 34,
  number       = 1,
  pages        = {64--73},
  publisher    = {Wiley Online Library},
  doi          = {10.1002/sapm195534164}
}