dot: make "a" the default

Fixes #319.

* spot/twaalgos/dot.cc: Enable "a" by default.
* bin/common_aoutput.cc, NEWS: Document it.
* doc/org/autfilt.org, doc/org/concepts.org, doc/org/dstar2tgba.org,
doc/org/hierarchy.org, doc/org/ltl2tgba.org, doc/org/oaut.org,
doc/org/randaut.org, doc/org/satmin.org, doc/org/tut23.org,
doc/org/tut24.org, doc/org/tut30.org, doc/org/tut31.org: Adjust or
simplify the documentation.
* tests/core/det.test, tests/core/dstar.test, tests/core/monitor.test,
tests/core/neverclaimread.test, tests/core/readsave.test,
tests/core/tgbagraph.test, tests/core/wdba.test,
tests/python/_autparserr.ipynb, tests/python/automata-io.ipynb,
tests/python/automata.ipynb, tests/python/highlighting.ipynb
tests/python/ltsmin-dve.ipynb, tests/python/ltsmin-pml.ipynb,
tests/python/product.ipynb, tests/python/testingaut.ipynb,
tests/python/word.ipynb: Adjust test cases.
20 jobs for next in 232 minutes and 52 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed arch-clang #2735

00:32:19

passed arch-gcc-glibcxxdebug #2736

00:33:43

passed debian-gcc-snapshot #2749

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passed debian-stable-gcc #2732

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passed debian-unstable-gcc-coverage #2733

00:25:24

88.9%
passed debpkg-stable #2751

01:42:31

passed debpkg-unstable #2740

01:40:23

passed mingw-shared #2745

00:09:41

passed mingw-static #2750

00:09:09

failed debian-gcc-snapshot #2748

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failed debian-gcc-snapshot #2744

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failed debian-gcc-snapshot #2742

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failed debian-gcc-snapshot #2734

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failed debpkg-stable #2747

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failed debpkg-stable #2743

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failed debpkg-stable #2739

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failed mingw-shared #2737

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failed mingw-static #2746

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failed mingw-static #2738

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  Publish
passed publish-unstable #2741
dput

00:01:26