Prepare release of v0.18.0

This MR prepares the v0.18.0 release of dantro.

Can this MR be accepted?

• Set version number in dantro/__init__.py
  • Removed the pre-release specifier.
  • Version is now: 0.18.0
• Prepared changelog for release
  • Removed "WIP" in section heading
  • If necessary, re-ordered and cleaned-up the corresponding section
• Drop minimum dependency check (see #270 (closed))
• Pipeline passes without warnings

Closes #270 (closed)