[FIX] VizRankDialog: Use extended thread pool to prevent segfaults

[VesnaT]

Issue

When fed large datasets to owcorrelations, the widget exited with segmentation fault, (probably) due to insufficient stack size for created task.

To reproduce: Generate a dataset with at least 26 features and 3361 rows, and feed it to OWCorrelations.

Description of changes

• VizRankDialog now inherits ConcurrentMixin, which uses ThreadExecutor that provides sufficient stack size.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3669 into master will increase coverage by 0.05%.
The diff coverage is 98.86%.

@@            Coverage Diff             @@
##           master    #3669      +/-   ##
==========================================
+ Coverage   84.43%   84.48%   +0.05%     
==========================================
  Files         372      373       +1     
  Lines       68087    68188     +101     
==========================================
+ Hits        57488    57609     +121     
+ Misses      10599    10579      -20

[ales-erjavec]

I get various

-------------------------- AssertionError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/utils/concurrent.py", line 954, in _on_task_done
    self.on_done(future.result())
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/utils/__init__.py", line 327, in on_done
    assert not self.state_count() or self.rank_model.rowCount() == self.state_count()
AssertionError

or

-------------------------- AssertionError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/utils/concurrent.py", line 954, in _on_task_done
    self.on_done(future.result())
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/visualize/utils/__init__.py", line 327, in on_done
    assert self.saved_state is None
AssertionError
-------------------------------------------------------------------------------

if I Pause/Continue multiple time during a run then let it run till the end in Correlations widget (testing on phoneme.tab).

[VesnaT]

I just realized these asserts should be removed because an exception can occur while computing the score and therefore is not put into the table.
But while testing phoneme.tab that is not the case, since all the scores can be computed. So apparently there is a bug related to pausing calculation.

[ales-erjavec]

Might I suggest just setting a larger initial stack size on the QThread instance, then maybe revisit this latter.

I think the ConcurrentMixin is not a good fit for this, at least as is.