ENH : better shrinkage range for ShrunkCovariance
FIX: unintended assignment to ValueError
ENH: use stable blas getter API
STY: tuple for Gael
ENH: refactor warnings 1
ENH: add assert_warn_message
ENH: revem tests_metrics.py
ENH: more of this
ENH: final fix: clear all regisitries
FIX: PPCA related warnings