Also introduce twa::unregister_ap() and twa_graph::remove_unused_ap()
so that the methods where this behavior is expected can be fixed.
And fix ltsmin::kripke() which did not register APs.
Part of #170.
* spot/twaalgos/hoa.cc: Use apvars() to print all registerd APs.
Throw an exception when printing automata using unregistered APs.
* spot/ltsmin/ltsmin.cc: Call register_ap().
* spot/twa/twa.cc, spot/twa/twa.hh, spot/twa/twagraph.cc,
spot/twa/twagraph.hh (twa::unregister_ap, twa_graph::remove_unused_ap):
New methods.
* spot/tl/exclusive.cc, spot/twaalgos/postproc.cc,
spot/twaalgos/remprop.cc, spot/twaalgos/relabel.cc: Use them.
* tests/core/maskacc.test, tests/core/maskkeep.test,
tests/core/strength.test: Adjust expected results.
* NEWS: Mention those changes.
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