Implement animated DAG-based errorbar function

This MR adds DAG-based errorbar and errorbands plotting functions, which (like facet_grid line plots) support hue and frames keywords for dimension selection.

Anything to double-check?

• Is the usage intuitive?
• Changes to PlotHelper and animation routine ok?
• %Version 0.13 release ok?
  • Reason: These plot functions would be useful right away in utopya, while the rest of #17 (closed) would presumably take more time. (Not the cleanest way to do it, admittedly. Looks like we had many afterthoughts to the 0.13 release 🙈 )

Can this MR be accepted?

• Implementation ready
  • errorbar and errorbands
  • When registering animation update, let PlotHelper store whether the helpers should be invoked prior to grabbing a frame
    • This got rid of visualisation problems encountered during animation, boiling down to the helpers not being applied to the re-drawn figure
  • __len__ for numeric data containers (needed to comply to matplotlib interface)
  • removed bad-practice mpl_setup module
• Tests added or adjusted
  • Basic tests
  • Legend handle and helper stuff works
• Documentation extended or updated
  • Docstrings
  • User manual entry

• 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 @jeremiastraub

Related issues

#17 (closed) (plans more function than this one, though, so: not closing)

#171 (closed) extension to support faceting