Stack: new table instead of transform

[ajdapretnar]

Issue

Fixes #6665.

Description of changes

Stacking transformed the data to the same data type as the input. However, Timeseries has an inbuilt assert for time variable, which was missing from stacked_data.

Tests will be added to Timeseries add-on.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6723 (83e3705) into master (7bdc8e4) will decrease coverage by 0.01%.
Report is 9 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head 83e3705 differs from pull request most recent head 3bb991d. Consider uploading reports for the commit 3bb991d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6723      +/-   ##
==========================================
- Coverage   88.13%   88.13%   -0.01%     
==========================================
  Files         322      322              
  Lines       70577    70577              
==========================================
- Hits        62206    62205       -1     
- Misses       8371     8372       +1     

[ajdapretnar]

[note to self] Converted to draft. Add a test creating a dummy Table class, then mocking self.aggregate from stack.py.

[ajdapretnar]

@janezd We went back and forth with Lan on this one. This was the simplest solution we could find.