At the same time, this adds a is_empty() method to the tgba class,
simplifying many places that ran emptiness checks.

* iface/dve2/dve2check.cc, src/bin/ltlcross.cc,
src/dstarparse/dra2ba.cc, src/ltlvisit/contain.cc, src/tgba/tgba.cc,
src/tgba/tgba.hh, src/tgbaalgos/emptiness.cc,
src/tgbaalgos/emptiness.hh, src/tgbaalgos/gtec/ce.cc,
src/tgbaalgos/gtec/ce.hh, src/tgbaalgos/gtec/gtec.cc,
src/tgbaalgos/gtec/gtec.hh, src/tgbaalgos/gv04.cc,
src/tgbaalgos/gv04.hh, src/tgbaalgos/magic.cc,
src/tgbaalgos/magic.hh, src/tgbaalgos/minimize.cc,
src/tgbaalgos/ndfs_result.hxx, src/tgbaalgos/powerset.cc,
src/tgbaalgos/projrun.cc, src/tgbaalgos/projrun.hh,
src/tgbaalgos/reducerun.cc, src/tgbaalgos/reducerun.hh,
src/tgbaalgos/replayrun.cc, src/tgbaalgos/replayrun.hh,
src/tgbaalgos/rundotdec.cc, src/tgbaalgos/rundotdec.hh,
src/tgbaalgos/se05.cc, src/tgbaalgos/se05.hh,
src/tgbaalgos/tau03.cc, src/tgbaalgos/tau03.hh,
src/tgbaalgos/tau03opt.cc, src/tgbaalgos/tau03opt.hh,
src/tgbaalgos/word.cc, src/tgbaalgos/word.hh,
src/tgbatest/checkpsl.cc, src/tgbatest/complementation.cc,
src/tgbatest/emptchk.cc, src/tgbatest/ltl2tgba.cc,
src/tgbatest/randtgba.cc, wrap/python/ajax/spot.in,
wrap/python/spot.i: Use shared_ptr.