Make Elastic-Net compilable

Things to do:

(1) Implement __gaussian_fista_state_diff and __gaussian_fista_result

(2) Update intercept_y

(3) Necessary updates in Python

(4) Add FISTA as an option on top layer

(5) Make FISTA and IGD work, and test their speed

More things to do:

* Grouping support, including grouping support for normalization

* Faster iteration for active set

* Completely repair CD optimizer