Bugfix release for PyPI

The main change in this release is completely refactored installation and setup, and distribution via PyPI.

[0.6.6] - Bugfix release - 2021-10-11

New features:

• added an explicit changelog
• experimental support for derivations shown using svgling trees
• full support for installation in site-packages, better kernel installation

Fixes, improvements, changes (highlights):

• improve derivation rendering
• improve cross-browser compatibility
• improve error messaging and handling of composition failures