ENH: Return attribute n_iter_ for linear models dependent on the enet solver
TST: Added test to check that warm model converges faster than a cold one
TST: Better to test that warm_start runs only once after the prev model has converged
TST: Added test to check that higher alpha converges faster
Added return_n_iter parameter for enet (and lasso) path
FIX: Fit the data in tests
Merge pull request #3349 from MechCoder/return_niter

[MRG+1] Return attribute n_iter_ for linear models dependent on the enet solver