integrating vowpal_wabbit's optimizers

[yorkerlin]

The existing vw modular in shogun is outdated and partially buggy :(.
As suggested by @karlnapf , we should do another round of integration.

In my opinion, there are two main components of vw to be integrated into shogun.

• distributed communication and IO
• (parallel) stochastic optimizers

Due to time constraint, I am mainly working on these optimizers. I assume that data can be completely loaded into memory using StreamingFileFromDenseFeatures.

[yorkerlin]

this is also related to issue
#3316

[karlnapf]

Absolutely,
I think focussing on the optimizers first is fine!