Gap filling and other S3 issues

[dajmcdon]

Reposting from @qpmnguyen via Slack:

When using epiprocess with tsibble, I have to switch back and forth between the two data types (epi_df and tsibble) in order to use their functionality (e.g. use epi_slide after gap filling)

[dajmcdon]

@brookslogan response:

Easier fixes might try to address particular bumps, e.g., provide our own gap-filling function, or epi_df implementations of tsibble S3 methods (that might just convert to tsibble, call tsibble implementation, convert back to epi_df). So maybe Quang could list any other situations that this happened and we could add epiprocess functions accordingly. Harder fixes and/or breaking changes might try to support tsibble-backed epi_dfs in addition to, or instead of, tibble-backed ones.

[dajmcdon]

related to #117

[brookslogan]

This might be a tool to use in #256; if we want to complete slide inputs, we may need a complete / gap-filling method for that works across both epi_dfs and tibbles (depending on whether our slide inputs have been decayed into tibbles or are still epi_dfs). tidyr::complete may or may not work for this already, but it's probably what we want to use. We're trying to avoid heavy tidyverts dependencies now.

[brookslogan]

I don't think there's an easy way to smooth out the tsibble stuff. What's left here is probably #250 + the extra issues in #223 &co.