target_Qbarbar: Target the iterated regressions
In benkeser/intermed:

Target the iterated regressions

target_Qbarbar(
  Qbar,
  Qbarbar,
  Y,
  A,
  a,
  a_star,
  gn,
  M1,
  M2,
  all_mediator_values,
  max_iter,
  tol = 1/(sqrt(length(Y)) * log(length(Y)))
)

`Qbar`	Outcome regression estimates
`Qbarbar`	Iterated mean estimates
`Y`	A vector of continuous or binary outcomes.
`A`	A vector of binary treatment assignment (assumed to be equal to 0 or 1).
`a`	The label for the treatment. The effects estimates returned pertain to estimation of interventional effects of `a` versus `a_star`.
`a_star`	The label for the treatment. The effects estimates returned pertain to estimation of interventional effects of `a` versus `a_star`.
`gn`	Power users may wish to pass in their own properly formatted list of the propensity score so that nuisance parameters can be fitted outside of `intermed`.
`M1`	A `vector` of mediators.
`M2`	A `vector` of mediators.
`all_mediator_values`	All combinations of M1 and M2
`max_iter`	The maximum number of iterations for the TMLE
`tol`	The tolerance for stopping the iterative targeting procedure.