Suchtree

Not much about SuchTree itself has changed, but the build system has been completely overhauled.
* Migrated build and unit tests from TravisCI to GitHub Actions
* Wheels for Linux on amd64 and MacOS on Intel silicon are now automatically built using [`cibuildwheel`](https://github.com/pypa/cibuildwheel)
* Dependencies should now be identified and installed correctly
* Various minor cosmetic fixes

* revised examples notebooks
* new example data for big trees
* added installation instructions
* fixed sundry typos in the documentation

Python3 support. New testing framework. Improved release engineering. Archived with Zenodo.

Added support for igraph output from SuchLinkedTrees!

Some new features and bug fixes to make SuchTree maybe actually useful.