Various improvements to the plotting framework

This MR adds various improvements to the plotting framework. See "Implementation" and "Documentation" checklist entries below.

Can this MR be accepted?

• Implementation ready
  • Allow configuring PlotHelper from ExternalPlotCreator and PlotManager
  • Add debug option to plot config, allowing to debug individual plots
  • Improve error message in PlotManager
  • Additionally ignore plot configs starting with . (_ already is ignored)
  • Fix a bug where the out_dir given in a single plot configuration was ignored
• Tests added or adjusted
• Documentation extended or updated
  • Document the debug flag and that both . and _ are ignored.
  • Extends the docs with a "plot config reference" using tested examples
  • Move plot creator auto detection entry to another file; is quite an advanced topic

• 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 @herdeanu

Related issues

Closes #160 (closed) , #161 (closed)