Resolve "Move custom groups and containers into a sub-module"

What does this MR do?

This MR improves modularisation of dantro by creating several new sub-packages and moving class definitions into separate modules, sorted by field of usage.

The new sub-packages are:

• containers (❗ s appended)
• groups (❗ s appended)
• mixins
• data_loaders
• proxy

All the modules in these sub-packages are made available at the package level, such that import paths remain (mostly) valid. Also, where possible (most places), imports from within dantro are made relative, to limit their scope.

Can this MR be accepted?

• Implemented changes
• Tests adjusted
• Pipeline passes
• Changelog entry added
• Bump version to 0.5.0-pre.1
• Take care of deprecation warnings (#28 (closed))
• Approved by @herdeanu

Anything to double-check?

Related issues

Closes #33 (closed), #28 (closed)