Plotting improvements: let PlotHelper skip empty axes, improve `based_on` error messages

This MR makes two minor improvements:

• PlotHelper now longer invokes a helper on an empty axis
  • Doing so led to errors in the past, e.g. if an empty axis is generated via col_wrap of facet_grid
  • Enabled by default, but can be disabled via skip_empty_axes flag
• PlotManager error message improvement for bad based_on value
  • Now also suggesting "Did you mean: ..?"
  • Now also using make_columns.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
  • Some unrelated tests relied on helpers being invoked even on unrelated axes; needed to adjust those
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @jeremiastraub

Related issues

Closes #230 (closed)