Add operation to unpack object arrays

This MR adds a data operation expand_object_array that expands an object-dtype array into a higher-dimensional array with numeric dtype. This is useful e.g. when loading variably-sized array-like data from HDF5, which is provided as an object-array of numerical arrays by h5py.

To accommodate this, some other changes are also made in this MR:

• Improve populate_ndarray operation
• Improve the corresponding call from within MultiversePlotCreator
• Improve the layout and ordering of the data operations database

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
  • Approved by @herdeanu