Add `make_facet_grid_plot` decorator and let errorbar support faceting

This MR:

• adds the make_facet_grid_plot decorator
• fixes bugs in determine_layout_encoding
• uses it to implement the errorbars plot which supercedes errorbar (but changes the interface)
  • deprecated errorbar in favour of errorbars deprecation
• improves the PlotHelper:
  • better modularisation
  • distinguish figure-level and axis-level helpers
  • add set_figlegend
  • let set_suptitle no longer create overlapping suptitles
  • minor bug fixes
• moves the plot helper exceptions to the main module

Anything to double check?

• errorbars working?
• other make_facet_grid_plot mappings are currently untested; ok to do this in another MR? (Far easier to do if implementing a plot function with it...)
• documentation ok?
• better idea for errorbar deprecation? Would otherwise be a breaking change (and quite immediate in Utopia, because we install directly from release/v0.15 there

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Full 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 @herdeanu
  • Reviewed & approved by @jeremiastraub

Related issues

Closes #171 (closed)