It was noted in #470 that make_twa_graph did not copy named
properties. Let's fix that.
* spot/twa/twa.hh, spot/twa/twa.cc (copy_named_properties_of): New
method.
* spot/twa/twagraph.hh (make_twa_graph): Add an extra argument to
call copy_named_properties_of() optionally.
* python/spot/__init__.py (twa_graph.__copy__): Use it.
* tests/python/twagraph.py: Test that.
* tests/sanity/namedprop.test: Ensure copy_named_properties_of copies
all known named properties.
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