[FIX] Fix transformation for non primitive variables

[ales-erjavec]

Issue

When editing a StringVariable in "Edit Domain" widget an TypeError is raised:

Traceback (most recent call last):
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/gui.py", line 1978, in do_commit
    commit()
  File "/Users/aleserjavec/workspace/orange3/Orange/widgets/data/oweditdomain.py", line 543, in commit
    new_data = self.data.from_table(new_domain, self.data)
  File "/Users/aleserjavec/workspace/orange3/Orange/data/table.py", line 374, in from_table
    is_sparse=sp.issparse(source.metas))
  File "/Users/aleserjavec/workspace/orange3/Orange/data/table.py", line 324, in get_columns
    a[:, i] = match_type(col(source))
  File "/Users/aleserjavec/workspace/orange3/Orange/preprocess/transformation.py", line 27, in __call__
    domain = Domain([self.variable])
  File "/Users/aleserjavec/workspace/orange3/Orange/data/domain.py", line 127, in __init__
    raise TypeError("variables must be primitive")
TypeError: variables must be primitive

Description of changes

Fix base Transformation class to handle the non primitive variable case.

Includes

• Code changes
• Tests
• Documentation

[janezd]

I fixed the same thing this weekend. :) See dbfa1e2.

The fix is the same, but tests are different. You call an actual transformation and I use a mock and call transform directly. Can you add my tests to your PR -- if you agree they make sense, of course? I would then merge your PR and remove my fix from my (otherwise unrelated) PR.

[codecov-io]

Current coverage is 88.91% (diff: 100%)

Merging #1770 into master will increase coverage by <.01%

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  Misses          981        981          
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