tgbaparse: Return a tgba_digraph.
* src/tgbaparse/parsedecl.hh, src/tgbaparse/public.hh,
src/tgbaparse/tgbaparse.yy: Adjust to return a tgba_digraph.
* src/priv/accmap.hh: New file to help creating acceptance
conditions from strings.
* src/priv/Makefile.am: Add accmap.hh
* src/tgba/tgbagraph.hh (tgba_digraph::named_t): New typedef.
* wrap/python/spot.i: Declare that tgba_digraph inherits from tgba.
* src/tgbatest/complementation.cc, src/tgbatest/explpro2.test,
src/tgbatest/explpro3.test, src/tgbatest/explpro4.test,
src/tgbatest/explprod.cc, src/tgbatest/explprod.test,
src/tgbatest/ltl2tgba.cc, src/tgbatest/maskacc.cc,
src/tgbatest/maskacc.test, src/tgbatest/mixprod.cc,
src/tgbatest/powerset.cc, src/tgbatest/randtgba.test,
src/tgbatest/readsave.test, src/tgbatest/tgbaread.cc,
src/tgbatest/tgbaread.test, src/tgbatest/tripprod.cc,
src/tgbatest/tripprod.test: Adjust to the change.
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