Various improvements to plotting with data transformation framework

This MR improves the procedures and performance when plotting with the data transformation framework. Specifically:

• Allow caching whole TransformationDAG trees in order to avoid re-building them (which can be very time-consuming if there are many nodes)
• Improve logging (formulation of messages, choice of level, removal)
• Improve communication of runtimes (only shown if beyond a threshold)
• Implement a significantly faster cPickle-based deep copy function which is used when building TransformationDAG
• Use __slots__ in DAG placeholder classes to reduce required memory
• In DAG, only load from cache file if the content was not already loaded

Bug fixes / robustness improvements:

• Let errorbar facet grid plot not use & character as default y-axis label, because that's a problematic character when LaTeX mode is enabled (related: #82)

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 @

Related issues