Add function for backfill analysis?

[dshemetov]

Moving an old todo note from archive.Rmd, after the paragraph

We can see some interesting and highly nontrivial revision behavior: at some
points in time the provisional data snapshots grossly underestimate the latest
curve (look in particular at Florida close to the end of 2021), and at others
they overestimate it (both states towards the beginning of 2021), though not
quite as dramatically. Modeling the revision process, which is often called
*backfill modeling*, is an important statistical problem in it of itself.

<!-- todo: refer to some project/code? perhaps we should think about writing a
 function in `epiprocess` or even a separate package? -->