Prepare release v0.13

This MR prepares the v0.13 release of dantro.

Can this MR be accepted?

• Set version number in dantro/__init__.py
  • Removed the pre-release specifier.
  • Version is now: 0.13.0
• Prepared changelog for release
  • Removed "WIP" in section heading
  • If necessary, re-ordered and cleaned-up the corresponding section
• Pipeline passes without warnings
  • If the test_minimal_deps stage creates warnings, inspect the output log and, if necessary, adjust the lower bounds of the dependencies in setup.py.