epi_df behavior for drop_na and pivot_*

[JavierMtzRdz]

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epi_df behavior for drop_na and pivot_* verbs

This PR includes methods for drop_na and pivot_* verbs.

drop_na

Always returns an epi_df, as it only subsets rows and does not remove essential key columns.

pivot_wider

pivot_wider method preserves the epi_df class as long as the geo_value and time_value columns are present in the output. It also automatically updates the other_keys metadata by removing any keys that were pivoted away or dropped during the transformation. However, the object will silently decay into a tibble if geo_value or time_value are used in names_from, as this removes them from the column set.

pivot_longer

The pivot_longer method automatically promotes the new names_to column to a key in the other_keys metadata. If the resulting data is not unique on its keys even after this promotion, the method will issue a warning and the object will decay into a tibble. Similar to pivot_wider, the result will silently become a tibble if the essential geo_value or time_value columns are pivoted away.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Partially resolves issue Patches for dplyr type conversions #391.

[JavierMtzRdz]

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[JavierMtzRdz]

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[JavierMtzRdz]

/preview-docs

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[JavierMtzRdz]

I have included the suggestions from your review! No relevant comments about it this time.