Round out dplyr&tidyr functionality for grouped `epi_df`s#310

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brookslogan wants to merge 2 commits intodevfrom

lcb/round-out-grouped-epi_dfs

Commits on Apr 27, 2023

Round out grouped `epi_df` ops using new `grouped_edf` class
lcbrooks
committed

Commits on May 1, 2023

Fix missing `group_trim` import, adjust some comments
lcbrooks
committed