[FLINK-1994] [ml] Add different gain calculation schemes to SG

[rawkintrevo]

Added 5 gain calculation schemes and .setOptimization method which takes an int values 0-5.

[tillrohrmann]

Hi @rawkintrevo, thanks for your contribution.

You have some scala style violations in GradientDescent.scala line 46 and 168. The lines mustn't be longer than 100 characters as the maven log output indicates.

Furthermore, could you change the title to the format "[FLINK-1994] [ml] Add different gain calculation schemes to SGD". That way, the JIRA issues will be automatically updated with the what happens on Github.

In Flink, we always rebase our changes on top of the current master branch. Therefore, it would be good if you could revert the last merge commit. If you want to update on the latest master, then do a rebase and a force push to the branch in your repository (here master).

[rawkintrevo]

did what you said ( I think ) and opened a new pull request