Changes from Mercurial (hg https://bitbucket.org/isa-afp/afp-devel/ default)
Summary
- adapted to Isabelle 80d7f004089d
- adapted to Isabelle 80d7f004089d
- merge from afp-2020
- Heptapod acknowledgements
- website for Safe_Distance
- new entry: Safe_Distance
- added missing HTML file
- sitegen for Nash_Williams
- new entry Nash_Williams
- metadata and sitegen for Smith-Normal-Form
- new entry: Smith normal form
- changed import to prepare for Smith-Normal-Form submission
- new entries Automated_Stateful_Protocol_Verification and Stateful_Protocol_Composition_and_Typing