This patch is mostly focused on tagging most virtual methods that
override as such.  I found a few methods that where not meant to be
virtual this way, and tagged a few classes "final" along the way.

* bin/common_output.cc, spot/kripke/fairkripke.hh,
spot/kripke/kripke.hh, spot/kripke/kripkegraph.hh,
spot/ltsmin/ltsmin.cc, spot/misc/formater.hh, spot/priv/bddalloc.hh,
spot/ta/ta.hh, spot/ta/taexplicit.hh, spot/ta/taproduct.hh,
spot/ta/tgta.hh, spot/ta/tgtaexplicit.cc, spot/ta/tgtaexplicit.hh,
spot/ta/tgtaproduct.hh, spot/taalgos/emptinessta.hh, spot/tl/declenv.hh,
spot/tl/defaultenv.hh, spot/tl/randomltl.hh, spot/tl/relabel.cc,
spot/twa/bdddict.cc, spot/twa/taatgba.hh, spot/twa/twagraph.hh,
spot/twa/twaproduct.hh, spot/twaalgos/gtec/ce.cc,
spot/twaalgos/gtec/ce.hh, spot/twaalgos/gtec/gtec.hh,
spot/twaalgos/gv04.cc, spot/twaalgos/ltl2taa.cc, spot/twaalgos/magic.cc,
spot/twaalgos/minimize.cc, spot/twaalgos/ndfs_result.hxx,
spot/twaalgos/reachiter.hh, spot/twaalgos/se05.cc,
spot/twaalgos/stutter.cc, spot/twaalgos/tau03.cc: Add more override and
final keywords.