[FIX] Group By - fix std and sum for TimeVariable

[PrimozGodec]

Issue

Fixe #6125

Description of changes

• use Series's std instead of GroupBy one since GB aggregations do not support DateTime data ENH: Support "std" aggregation in groupby for groupby datetime column pandas-dev/pandas#48481
• for variance, first, transform DateTime to Unix epoch since Panda's variance does not support DateTime
• std and span transform TimeDelta to seconds (Orange does not have TimeDelta type)
• I disabled the sum for TimeVariable since it doesn't make sense to compute data/times' sums.

Includes

• Code changes
• Tests
• Documentation

[PrimozGodec]

/rebase

[codecov]

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%           
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  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.