This function figures out which rows are missing in either state and applies
the provided enter and exit functions to fill in the blanks and provide
a 1-to-1 relation between the rows in from and to.
.complete_states(from, to, id, enter, exit, max_id)A list with the elements from and to holding the filled out
versions of from and to
Data.frames to tween between
The name of the column that holds the matching id
functions to fill out missing rows in from and to
respectively