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.

NEWS

@@ -13,6 +13,10 @@ New in spot 2.3.0.dev (not yet released)
 
   - spot::twa_run::as_twa() has an option to preserve state names.
 
+  - the method spot::twa::is_alternating(), introduced in Spot 2.3 was
+    badly named and has been deprecated.  Use the negation of the new
+    spot::twa::is_existential() instead.
+
   Bugs fixed:
 
   - spot::otf_product() was incorrectly registering atomic