FreeViz: Allow setting ratio btw attractive and repulsive forces

[janezd]

Issue

Fixes #6479.

Description of changes

Adds an option to control the strength of attractive and repulsive forces.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #6515 (1947679) into master (ca5bac8) will increase coverage by 0.07%.
Report is 34 commits behind head on master.
The diff coverage is 71.42%.

❗ Current head 1947679 differs from pull request most recent head 8bb82de. Consider uploading reports for the commit 8bb82de to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6515      +/-   ##
==========================================
+ Coverage   87.69%   87.77%   +0.07%     
==========================================
  Files         321      321              
  Lines       69420    69451      +31     
==========================================
+ Hits        60878    60959      +81     
+ Misses       8542     8492      -50     

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[processo]

Just wanted to add that with this new code changing Initialization while Gravity checked either does nothing or gives the error:

line 293, in init_projection
  gravity=self.gravity if self.balance else None
AttributeError: 'OWFreeViz' object has no attribute 'gravity'

[VesnaT]

Iris -> Freeviz -> Start -> Check Gravity -> select Random Initialization

-------------------------- AttributeError Exception ---------------------------
Traceback (most recent call last):
  File "/Users/vesna/orange-widget-base/orangewidget/gui.py", line 2261, in __call__
    self.func(**kwds)
  File "/Users/vesna/orange3/Orange/widgets/visualize/owfreeviz.py", line 231, in __init_combo_changed
    self.init_projection()
  File "/Users/vesna/orange3/Orange/widgets/visualize/owfreeviz.py", line 293, in init_projection
    gravity=self.gravity if self.balance else None)
AttributeError: 'OWFreeViz' object has no attribute 'gravity'
-------------------------------------------------------------------------------

[VesnaT]

Adding some tests for the new code would be nice.
Also, lint does not pass.