Fix epi_df creators

[dajmcdon]

epiprocess/R/epi_df.R

Line 114 in a509f03

#' @export

• as_epi_df() does the validation and wraps new_epi_df(), so that's all the user would need.
• new_epi_df() should not be exported unless it validates it's arguments.
• as_epi_df() needs to validate that any column names in additional_metadata$other_keys are in x.

(We may also want to consider making other_keys a specific argument rather than forcing it to be part of the additional_metadata list.)

[brookslogan]

Think this is basically[Related:] #301 + #186 but probably good to have it near the top of the issue list. [Also related: #166]