Make `autoplot.epi_archive` behave better when multiple value columns exist

[brookslogan]

Because plots automatically pop up, the warning about variable autoselection can be easily be overlooked.

Consider one of these options:

• Adding an axis or facet label with the selected variable name
• Plotting all value columns by default (using faceting) (Fix some minor bugs in autoplot.epi_archive #667 would need fixed first)
• Refusing to automatically select a variable if there are multiple candidates (as in current revision_summary())