Adaptations necessary for deeevolab migration

In order to smoothly transition deeevolab from the deval package to dantro, some minor adaptations to dantro became necessary.

These should not interfere with other parts of dantro, but be seen more as an enhancement and addition of minor features. All additions and changes are tested.

What does this MR do?

• Base objects:
  • Add an ObjectContainer class, which is a general container to store any object
  • Make _format_info a function that is not added as part of a mixin but already in the BaseDataGroup
  • Lift the requirement of item access having to be a string; now also allow passing lists, which is the internally used object. This not only reduces string split and list join operations, but also allows passing on non-string keys to the leafs of the data tree.
• Data loading:
  • Make it possible to pass a base load configuration via class attributes
  • Add a PickleLoaderMixin to load pickled objects; they are generated as ObjectContainers
  • Adjust the BaseDataManager to allow loading entries into the attrs of a data container or group
    • This happens via the key load_as_attr, which performs regular loading but then stores the object in the attributes of the already existing object at target_path
• Plotting:
  • Ignore plot entries that start with _; this allows to specify yaml shortcuts in the same file
• Minor cleanup, re-formatting, and improving documentation

Can this MR be accepted?

• Implemented changes
• Tests written and/or adjusted
• Pipeline passing
• Added changelog entry
• Bumped version number