Upgrade IGD for elastic net (gaussian)

Use the mirror truncation to get sparse coefficients.

Other changes in IGD for elastic net:

* changed model to coef
* use a separated intercept term in task
* added variables that are needed by mirror truncation to task

To do for IGD:

* To change SQL and Python part so that they are compatible with the new
  state definition
* To change the computation of intercept since it is better handled now
* To add array[...] support