Consider adding join/merge between `epi_archive` and data frame

[brookslogan]

Add a join/merge function to combine an epi_archive with non-versioned data in a data frame, e.g., to:

• add unversioned population data
• add unversioned epi data from an epi_df
• potentially, add unversioned data without a geo_value from a data frame

Naming might need thought, since R6 $merge is already taken with S3 epix_merge.

[brookslogan]

The existing $merge allows for this, but does not document it, and probably leaves the archive DT with an invalid key. The new $merge in #101 does not allow for this (but a user could hack it by merging with an epi_archive considering all data observed in 1000BCE and nonclobberable).