Changed default argument of precompute in ElasticNet and Lasso

Setting precompute to "auto" was found to be slower when n_samples > n_features
since the computation of the Gram matrix is computationally expensive and
outweighs the benefit of fitting the Gram for just one alpha.
TST: Added tests to test Deprecation warning
DOC on negation of loss functions used as scoring parameters

+ copy editing to model evaluation narrative documentation
DOC reduce table width through less-qualified function names