Allow skipping plot if file already exist

This MR adds a small tweak to BasePlotCreator that allows skipping a plot if the file already exists. This option is useful because otherwise only raising (with an error) or overwriting are possible.

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
  • Reviewed & approved by

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