deprecate copy() in favor of make_twa_graph()

Fixes #258.

* spot/twaalgos/copy.cc: Delete, and move the code...
* spot/twa/twagraph.cc: ... in some anonymous namespace here.
* spot/twa/twagraph.hh: Adjust the make_twa_graph() overload.
* spot/twaalgos/copy.hh, NEWS: Mark copy() as deprecated and redirect
to make_twa_graph().
* doc/org/upgrade2.org, doc/org/tut51.org, python/spot/impl.i,
spot/twaalgos/dot.cc, spot/twaalgos/langmap.cc, tests/core/ikwiad.cc:
Adjust callers.
* spot/twaalgos/Makefile.am: Remove copy.cc.

spot/twaalgos/Makefile.am

@@ -98,7 +98,6 @@ libtwaalgos_la_SOURCES = \
   complete.cc \
   complement.cc \
   compsusp.cc \
-  copy.cc \
   cycles.cc \
   degen.cc \
   determinize.cc \