game: rewrite, document, and rename solve_reachability_game

* spot/twaalgos/game.hh, spot/twaalgos/game.cc: Rename
solve_reachability_game() as solve_safety_game(), rewrite it (the old
implementation incorrectly marked dead states as winning for their
owner).
* tests/python/paritygame.ipynb: Rename as...
* tests/python/games.ipynb: ... this, and illustrate
solve_safety_game().
* tests/Makefile.am, NEWS, doc/org/tut.org: Adjust.
* tests/python/except.py: Add more tests.
16 jobs for next in 193 minutes and 59 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed alpine-gcc #117621

01:43:08

passed arch-clang #117622

01:28:32

passed arch-gcc-glibcxxdebug #117623

01:17:20

passed debian-gcc-snapshot #117620

02:49:36

passed debian-stable-gcc #117618

01:08:43

passed debian-unstable-gcc-coverage #117619

02:00:38

90.6%
passed debpkg-stable #117626

01:25:05

passed debpkg-unstable #117627

02:05:16

passed mingw-shared #117624

00:32:26

passed mingw-static #117625

00:19:24

passed raspbian #117629
armv7

01:30:55

passed rpm-pkg #117628

01:26:25

 
  Build2
passed debpkg-stable-i386 #117630
x86

01:14:42

passed debpkg-unstable-i386 #117631
x86

01:06:17

 
  Publish
passed publish-rpm #117632
dput

00:00:28

passed publish-unstable #117633
dput

00:01:51