TST make test more stable.
DOC + FIX density to Ping and al. recommandation
ENH make jl lemma work even with non numpy array
DOC add default values
Updates for DBSCAN clsutering docs

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Couple of small changes from comments
ENH slight improvement of common tests.
Add failing test and no-op flip
Sign flipping as suggested by @ogrisel, not in place
Make sign flip in place
Test more seeds for svd sign flipping
Add sign flip as flag in randomized_svd
Make sure svd_flip test actually tests something
Make randomized_svd flipped by default
svd_flip test fails on Travis. Change random seed, see if it helps
Cannot easily ensure non-uniqueness without the fix, just test uniqueness
TEST flipped svd remains correct
DOC slight cosmit in metrics docstrings.
FIX i should trust my past self a bit more
ENH use an array instead of a dict in RFECV