reorder_list: Helper function to reorder lists according to cvFolds
In benkeser/intermed:

Helper function to reorder lists according to cvFolds

reorder_list(
  a_list,
  a_0,
  valid_rows,
  n_SL = 1,
  which_nuisance,
  n,
  n_mediator_values,
  n_M1_values = 0,
  n_M2_values = 0
)

`a_list`	Structured list of nuisance parameters
`a_0`	Treatment levels
`valid_rows`	List of rows of data in validation data for each split.
`n_SL`	Number of super learners. If >1, then predictions are averaged over repeated super learners
`which_nuisance`	Structure of reordering is slightly different for each relevant nuisance parameter ("OR", "PS", "MED")
`n`	Sample size
`n_mediator_values`	Number of unique combinations of mediators