[FIX] Transform: Replace 'Preprocess' input with 'Template Data' input

[VesnaT]

Issue

Fixes #3623 and #3675.

Description of changes

• remove 'Preprocessor' output from projection widgets (PCA, tSNE, Freeviz, Radviz, LinearProjection)
• OWTransform: use domain to transform data (instead of preprocessor)

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3673 into master will decrease coverage by <.01%.
The diff coverage is 98.01%.

@@            Coverage Diff             @@
##           master    #3673      +/-   ##
==========================================
- Coverage   84.35%   84.34%   -0.01%     
==========================================
  Files         370      370              
  Lines       67954    67903      -51     
==========================================
- Hits        57321    57272      -49     
+ Misses      10633    10631       -2

[VesnaT]

Since OWTransform now makes transformation using arbitrary domain, we should probably reconsider #3526.

[janezd]

Is this still WIP?

What about renaming the widget - do we have a better name?

I agree we should reconsider #3526.

[VesnaT]

It's WIP because of #3526.
I don't know about the name. I wouldn't change it until we completely agree on the new one.
Besides, any ideas on what to do with the (removed/replaced) signals to make saved workflows work?

[janezd]

#3526 can be dealt with later.

Name: let's find one tomorrow.

What about not caring about saved workflows in this case? The widget was not around for long so it couldn't had been used extensively, while supporting this, even if perhaps possible (though not entirely clear how), would include too much dead code.