Fixes Bug #0013433 - allow ancestor_too_old() tests to be done

[CameronD73]

When calling probably_alive, the previous code did not clear the self.pset list between running descendant test and ancestor test, so the ancestor test code assumed all persons had already been checked and was never executed.

This bug and fix is easier to evaluate if patch for bug #13431 has been applied - that is just to improve debug output.

[CameronD73]

The failures in the unit tests are probably expected as a result of the bug fix

• The geneweb export test added a zero death date to indicate known death but unknown date because the bug fix now does the ancestor test, which returns "probably dead by 2019" for I0042, since his parents were born 1889. However his only record is being adopted (with no date), so the assumptions behind the calculations are to some extent not applicable
• The "probably alive in 1900" test returned quite a few differences
  -- 4 who should have been tagged as alive but were not in the released version 5.2.3
  -- 45 who were tagged as probably alive in 1900 in released ver 5.2.3 but were very unlikely alive. Some were definitely wrong in 5.2.3, some might be alive by a long stretch, but had very little associated detail, One was incorrect in the proposed bug fix.