Resolve "Support multiple base plot configurations"

This MR ...

• revamps and generalises the based_on functionality (#247 (closed))
  • now specifying base_cfg_pools: multiple plots configurations from which the lookup can happen
  • lookup within the same plots configuration is possible
  • recursive lookup is possible
• adds the dantro.plot module (groundworks for #236 (closed))
• cleans up and tweaks minor parts of the PlotManager (#252 (closed))
  • log messages
  • no longer necessary code

Anything to double-check?

Working as expected? ... this is probably best tested via utopya

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated
  • docstrings
  • user manual

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • #260 (closed) Follow-up issue for deprecation of base_cfg and update_base_cfg created
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved

Related issues

Closes #247 (closed), #252 (closed)