Details about how tsinfer does imputation

[hyanwong]

@chlangley asked how tsinfer does imputation. This isn't actually documented anywhere, so it would be good to have a documentation section on this, with a few details, e.g. in the "inference overview" section. Any text in the discussion below can be used to fill out the docs.

[chlangley]

Found it. Thanks.

[hyanwong]

Incidentally, to replace imputed data with the original missing data, we would need to address #224

[hyanwong]

Pinging @szhan as he might start to write a tutorial on this 🎉

[hyanwong]

(Sadly there's no material posted here yet, but this GH discussion could serve as a place to talk about it.)