double_gauss_DoG

  DOUBLE_GAUSS_DOG - computes a double-gaussian function that is modulated
  by a difference of gaussians

  Z = DOUBLE_GAUSS_DOG(THETA, SF, DOUBLE_GAUSS_PARAMS, DOG_PARAMS)

  THETA corresponds to angle
  SF corresponds to spatial frequency
  DOUBLE_GAUSS_PARAMS = [a b c d e]
  DOG_PARAMS = [0 a2 b2 c2 d2]

  See also: DOUBLE_GAUSS_180, DOG