Improve PlotFuncResolver customisability

Adds the PlotManager._get_plot_func_resolver method, which allows more convenient subclassing, e.g. if desiring to initialise the resolver with additional information from the PlotManager.

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