Parallelise ParamSpace-based plots

This started out a bit experimental ... but works actually really nice for some tasks. There's a bit of an overhead in spawning processes or threads, so it is only be useful for plots that take a bit longer (>3-5s or so).

Uses concurrent.futures.

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 (already bumped very recently)