[ENH] Import documents: Import from URL

[VesnaT]

Issue

Description of changes

• introduce URL reader
• add URL option to Import Documents widget

Includes

• Code changes
• Tests
• Documentation

[codecov-io]

Codecov Report

Merging #637 (6ca6394) into master (4c4069b) will increase coverage by 0.31%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #637      +/-   ##
==========================================
+ Coverage   71.29%   71.60%   +0.31%     
==========================================
  Files          66       66              
  Lines        7806     7939     +133     
  Branches     1027     1037      +10     
==========================================
+ Hits         5565     5685     +120     
- Misses       2038     2048      +10     
- Partials      203      206       +3     

[PrimozGodec]

One more comment that we do not forget: this implementation is using add_column on a table. We need to raise the minimum version of Orange before it is merged to 3.28 (if I am right it is the first version supporting add_column).

[VesnaT]

True, I'll raise the version.

[ajdapretnar]

Should .yaml also be specified here?

[VesnaT]

No, it should not. These formats are 'documents' formats. Each file of these format represents a document. Metadata is considered differently, since it appends data to each document. Its formats are hardcoded in _read_meta_data().

[ajdapretnar]

I noticed there's a YAML reader and a function that should recognize metadata. But I don't see how this is reflected in the widget.

[VesnaT]

It is not. It just reads the metadata, if there is any. Do you think we should add something to the GUI? What do you propose?
Maybe the name of the KEY column (the one that connects the metadata and the document).