Merging map units through merge_by or summarize
changes the indexing of each unit. Use this function to take a set of
redistricting plans from a redist algorithm and re-index them to
be compatible with the original set of units.
pullback(plans, map = NULL)a redist_plans object
optionally, a redist_map object, which will be used to set the new population vector
a new, re-indexed, redist_plans object