class for ec_statistics and ars_statistics.
(acss_statistics): Inherit from ars_statistics.
* tgbaalgos/emptiness.cc, tgbaalgos/emptiness.hh:
(emptiness_check::statistics, emptiness_check_result::statistics):
New methods.
* tgbatest/randtgba.cc: Adjust to use the above.
* tgbaalgos/gv04.cc, tgbaalgos/ndfs_result.hxx, tgbaalgos/gtec/ce.cc,
tgbaalgos/gtec/ce.hh: Do not inherit from ars_statistics if
acss_statistics is used.