add measurement field

[dsweber2]

Came out of a discussion with @lcbrooks about how to get the column names of an epi_df that aren't keys/are measurements. Currently the best way to do this is

    cols <- names(epi_data)
    cols <- cols[!(cols %in% c("geo_value", "time_value", attr(epi_data, "metadata")$other_keys))]

which is a bit of a hack. Needed this a couple of times in exploration-tooling.