is_alternating() -> !is_existential()

Part of #212.

* spot/misc/common.hh (SPOT_DEPRECATED): Improve support current
compilers and options flags.
* spot/twa/twagraph.hh, spot/graph/graph.hh (is_alternating): Mark it
as deprecated.
(is_existential): New method.
* bin/autfilt.cc, bin/ltlcross.cc, spot/parseaut/parseaut.yy,
spot/twa/twa.cc, spot/twa/twagraph.cc, spot/twaalgos/alternation.cc,
spot/twaalgos/are_isomorphic.cc, spot/twaalgos/canonicalize.cc,
spot/twaalgos/couvreurnew.cc, spot/twaalgos/cycles.cc,
spot/twaalgos/degen.cc, spot/twaalgos/determinize.cc,
spot/twaalgos/dot.cc, spot/twaalgos/dtbasat.cc,
spot/twaalgos/dtwasat.cc, spot/twaalgos/hoa.cc,
spot/twaalgos/isunamb.cc, spot/twaalgos/isweakscc.cc,
spot/twaalgos/mask.hh, spot/twaalgos/minimize.cc,
spot/twaalgos/postproc.cc, spot/twaalgos/product.cc,
spot/twaalgos/randomize.cc, spot/twaalgos/remfin.cc,
spot/twaalgos/sbacc.cc, spot/twaalgos/sccfilter.cc,
spot/twaalgos/sccinfo.cc, spot/twaalgos/simulation.cc,
spot/twaalgos/strength.cc, tests/core/graph.cc, tests/core/ngraph.cc,
tests/python/alternating.py: Adjust all uses.
* NEWS: Mention the renaming.

spot/parseaut/parseaut.yy

@@ -2436,7 +2436,7 @@ namespace spot
     fix_acceptance(r);
     fix_initial_state(r);
     fix_properties(r);
-    if (r.h->aut && r.h->aut->is_alternating())
+    if (r.h->aut && !r.h->aut->is_existential())
       r.h->aut->merge_univ_dests();
     return r.h;
   };