Grapher: Add an option to use custom numerical labels in the data table

[sophiamersmann]

Brought up by Fiona on Slack

Core problem

• Some maps with numerical data define custom labels (using customNumericLabels)
• Usually we want to show the original values in the data table, but sometimes authors prefer to show the custom labels
  1. Authors sometimes map categorical data to numerical values (in order to show a line chart), and the use customNumericLabels to use the category labels for the map – in this case, showing the numerical values in the data table isn't helpful
  2. Fiona mentions this chart, with the following note: "The alternative would be to set the values in the ETL, but then I think the legend would then look like the standard categorical one, e.g., here, which I don’t think would be as effective."

Proposed solution

Case (i) should probably be solved by adding better support for categorical data (rather than adding more patches for the workaround)

For case (ii) we could be adding a new config field (although it might not be worth it if it's only this one chart that would benefit - given that we solve case (i) in a different way)

[danyx23]

We'd prefer to solve this with a better display of categorical data over time (see #4178)