counter_example. And we print the prefix and the periode of
counter_example's result.

* src/tgbatest/emptinesscheckexplicit.cc (main):
We call tgba_emptiness_check before counter_example.
* src/tgbatest/emptinesscheck.cc (main):
We call tgba_emptiness_check before counter_example.

* src/tgbaalgos/emptinesscheck.hh (spot):
(spot::print_result): New methode to print the prefix and the
periode of counter_example's result.

* src/tgbaalgos/emptinesscheck.cc (spot): counter_example doesn't
call tgba_emptiness_check. counter_example must be executed after
calling tgba_emptiness_check.  Remove tgba_emptiness_check calls.
(print_result): New methode to print the prefix and the
periode of counter_example's result.  Remove most of all std::cout
during execution of emptiness_check's methodes.