Address concurrency and cache-reading issues with forced cache read/write

This MR aims to address issues raised by @peanutfun in #342 (closed).

• Allow not to unpack loaded data cache objects (i.e. return cont.data instead of the cont the data was loaded into).
  • This is not a long-term solution, but a temporary workaround to customise whether loaded objects should again be dantro containers. In the long run, this requires more intelligent saving of cache files that allow restoring into exactly the type that they were before.

    read:
      enabled: true
      always: true
      load_options:
        unpack: false
        chunks: {...}
        # ... further options

• Fixes a bug where the DataManager acted as an unwanted cache layer despite being configured to always read from file
• Concurrency issues when using dask Cache file conflicts, see #342 (closed)
• Fixes a bug where allow_overwrite was ignored with the write.always flag being set.

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

Related issues

Closes #342 (closed)