"Promote" other_keys to be printed, a constructor parameter, and more clearly documented

[brookslogan]

Migrated from #113 and #183.

• print.epi_df should print the other_keys metadata.
• other_keys should be its own parameter to new_epi_df and as_epi_df
• Unfortunately, I don't think we ever explained what "keys" were in the context of an epi_df, and googling "tibble key" is surprisingly unhelpful (currently giving links to lock picks and decoder tools) [so the {new,as}_epi_df docs may need explication]. Maybe explaining this could just be rolled into the issue for adding other_keys as a parameter, though; it'd be easier to do in its own @param.

If this is causing recurring issues in epipredict maybe it should be changed to higher priority.

[brookslogan]

We should probably also make other_keys=character(0) the default. [Completed in #390.]

[brookslogan]

And we should also print the rest of the epikey in epi_archive's print method as well (e.g., after the geo and time type).

[brookslogan]

We need to regenerate the saved data sets (here and in other repos) after this change, or maybe right now due to the other_keys default change, or add notes in implementation to try to maintain backwards compatibility with saved edf's, or...

[dsweber2]

When "you say after this change", are you referring to the default of other_keys switching to character(0)?