ENH : add reconstruction_err_ for NMF with sparse input
use scipy.linalg in test_nmf.py
adding comment on why sparse frobenius is ok as done
Merge pull request #1607 from agramfort/reconstruction_err_nmf_sparse

MRG : add reconstruction_err_ for NMF with sparse input
DOC: small fix in the regression's score method documentation
Merge branch 'master' into develop