Expand `multiplot` and improve PlotHelper

This MR ...

• expands the multiplot function to allow making plots on multiple subplots
  • also adds a syntax for on-the-fly import of a callable: function: [module, name]
• improves the PlotHelper
  • adds the annotate helper, which complements the set_texts helper
  • adds the invoke_functions helper, which provides similar capabilities as the multiplot function, but via the PlotHelper interface.
  • adds a shorter syntax to specify axis-specific updates, now no longer requiring the axis key
• adds a workaround for #82 to address #261 (closed)

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• 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
  • (Partly) reviewed by @herdeanu

Related issues

#94

Closes #261 (closed) (workaround for #82)