Resolve "Implement auto-encoding of hue, col, row, frames … specifiers in generic plot functions"

This MR implements auto-encoding for layout specifiers (x, y, col etc.) and automatically selecting a plot kind.

Details

For that purpose, two free functions are implemented, determine_plot_kind and determine_layout_encoding. These inspect the given data and choose a plot kind and layout encoding according to the data dimensionality and the available encoding specifiers for the chosen kind. For more information, see the respective docstrings.

Anything to double-check?

See open discussions.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated
  • docstrings
  • facet grid documentation
  • examples will be done elsewhere, see #201 (closed)

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code: got full coverage.

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

Related issues

Closes #214 (closed)