Gradient descent small updates and fixes

[PrimozGodec]

With this pull request we updated:

• Modified gradient color to match to class.
• Fixed save image functionality
• Added some more information to the report.

[codecov-io]

Current coverage is 98.24% (diff: 100%)

Merging #19 into master will increase coverage by 0.02%

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  Files            13         13          
  Lines          1576       1595    +19   
  Methods           0          0          
  Messages          0          0          
  Branches        184        186     +2   
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+ Hits           1548       1567    +19   
  Misses           17         17          
  Partials         11         11          

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[ajdapretnar]

I like the current improvements, however, I do have some comments.
Could reported parameters be in a new lines below the graph (as they are in the Scatter Plot widget)?
Also, is it possible to include X and Y parameters in the reported graph, rather than in the description below? I really wish to have the same graph in Report as we have it in Save Image (X and Y axis description, final cost and maybe even colors (but that's optional).

Everything else looks good to me!

[PrimozGodec]

It is fixed now via biolab/orange3#1541

[ajdapretnar]

I'll check once again and I'll merge.