Clean up downloaded data tables

[sophiamersmann]

Description

If a chart has tolerance, then the filtered data table download has the columns time and year, which is a bit confusing.

Expected behaviour

Either transform the table to have a single time/year column, or improve the column names and documentation.

Steps to reproduce

1. Go to https://ourworldindata.org/grapher/economic-inequality-gini-index
2. Note that almost every country uses a value from a different year
3. Open the download modal and download the filtered data

Additional context

I don't think we prettify the tables for download in any way, so there might be similar gotchas for different charts

[marcelgerber]

Some of these columns are indeed quite confusing, and it's also odd that the Year and Code columns are only present for some rows, not all.

We should post-process the table before we convert it to a csv.