Resolve "Add plain text data loader"

What does this MR do?

This MR implements the possibility to load plain text files. For this, a StringContainer is implemented and a function loading a text file into a dataset.

Can this MR be accepted?

• Implement the functionality to load in plain text
  • Implement a StringContainer in dantro/container/general that stores string data is derived from a CollectionMixin and a PassthroughContainer
  • Implement a _load_plain_text function within a newly created dantro/data_loaders/load_text.py file.
  • Add the plain text loader to the dantro data loaders
• Tests added or adjusted
  • Add a StringContainer test
  • Add a TextLoaderMixin test
  • 100% test coverage on new code
• Pipeline passes
• Changelog entry added
• Version bumped to 0.11.0a2
• History cleaned-up
• Approved by @blsqr

Anything to double-check?

• Are the tests sufficient? (from resolved discussions)

Related issues

Closes #55 (closed)