Group By - fix std and sum for TimeVariable

[PrimozGodec]

Issue

Fixe https://github.com/biolab/orange3/issues/6125

Description of changes

Since Pandas' std fails on GroupBy, I temporarily replaced it with DataFrame's and reported a bug https://github.com/pandas-dev/pandas/issues/48481. I disabled the sum for TimeVariable since it doesn't make sense to compute data/times' sums.

Todo

• [ ] Timebomb for fixes in pandas
• [ ] Discuss what to do with the TimeDelta result (by std, var, span) since orange does not have the TimeDelta variable. Should it be transformed into seconds?
• [ ] Results from var operation are suspicions. Check them.

Includes

• [X] Code changes
• [x] Tests
• [ ] Documentation

[PrimozGodec]

/rebase

[codecov[bot]]

Codecov Report

Merging #6133 (2400565) into master (2979b21) will increase coverage by 0.00%. The diff coverage is 95.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6133   +/-   ##
=======================================
  Coverage   86.48%   86.48%           
=======================================
  Files         315      315           
  Lines       67774    67797   +23     
=======================================
+ Hits        58616    58637   +21     
- Misses       9158     9160    +2     

[VesnaT]

There is a problem with saved settings. If the Sum had been checked, it cannot be unchecked (it's disabled). This should be handled in settings migration.