Add line list -> epi_df, epi_archive utilities

[brookslogan]

Simple versions are probably a day; considering where NAs, >2 time labels, >2 event types, >1 signal, etc. and considering removals in -> epi_archive might make more work. @XuedaShen has code for one application.

Removals may be encoded as a removed_date column, or relabelings, updates, & removals may be reflected in multiple rows for the same event/id.

[nmdefries]

Context on Slack