Changes

Summary

1. more NEWS;
2. merged
3. avoid memory problems on test machine;
4. more robust defaults: spurious problems with parallel invocations and interrupts;
5. more interrupts;
6. avoid multiple uses of the same ML file;
7. tuned document, notably authors and sections;
8. clarified presentation of files for each theory;
9. disable auto_nitpick for now: spurious problems with non-termination e.g. in HOL-Hoare examples;
10. clarified modules: avoid multiple uses of the same ML file; clarified concrete vs. abstract syntax;
11. clarified session: avoid merge of different syntax from different Hoare logics;
12. clarified modules: avoid multiple uses of the same ML file;
13. clarified syntax modes, avoid obsolete "xsymbols";
14. clarified fonts, notably for Windows L&F;
15. support jdk-15;
16. more friendly desktop application on macOS;
17. default simprule for geometric series
18. tuned