add measurement field

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.