This function takes in a dataframe that is result of post_process_df (or track_components)
and an integer t (total time points), and returns a dataframe with 4 columns:
count, added, deleted, and survivor. count is the number of cells at time point t.
added is the number of new cells in time point t (relative to time point t - 1);
added is always NA for t = 1.
deleted is the number of cells at t that do not show up at later time points.
survivor is the number of cells at time point t that remain from time point 1.