[ENH] OWSGD: Output coefficients

[VesnaT]

Issue

Output coefficients for SGD classification and regression widgets

Description of changes

Use Orange preprocessors instead of sklearn Pipeline

• fixes: https://sentry.io/biolab/orange3/issues/210510912/

Modify OWSGD ancestor to output coefficients

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #1981 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master    #1981      +/-   ##
==========================================
+ Coverage   89.45%   89.46%   +<.01%     
==========================================
  Files          90       90              
  Lines        9182     9189       +7     
==========================================
+ Hits         8214     8221       +7     
  Misses        968      968

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[lanzagar]

Using Orange's preprocessors instead of sklearn Pipeline could result in a simpler fix.
Could you look into it?

[VesnaT]

Will do.