Add DAG `define` interface and `transform_after_combine` for MultiversePlotCreator

This MR adds improvements to the DAG framework as well as multiverse plotting:

For transformation framework in general: define interface added

... as discussed in #272 (comment 65054)

    define:
      exponent: 4               # directly define some object
      days_to_seconds_factor:   # use a sequence of TransformationsDAG
        - expression: "60 * 60 * 24"
        - float
    select:
      some_data: path/to/some_data
      more_data: path/to/more_data
    transform:
      - add: [!dag_tag some_data, !dag_tag more_data]
      - mul: [!dag_prev , !dag_tag days_to_seconds_factor]
      - print
      - pow: [!dag_prev , !dag_tag exponent]
        tag: my_result

For MultiversePlotCreator:

• Add transform_after_combine, very useful for apply operations to all fields (e.g. squashing out left-over dimensions which make plotting more difficult)
• Allow custom combination method (additionally to concat and merge)

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 to 0.16.3
  • Reviewed & approved

Related issues

#272 (closed) (but not closing because not fully implementing that)