Fix bug 642: add vectorization of sqrt for doubles, and make sqrt really safe if EIGEN_FAST_MATH is disabled
(grafted from 97d9a77721069028ca3bbe3c4d8978caeed940ac and 888c4f9a9c65918d8809e2ba51df88b7b26ecd77)

--HG--
branch : 3.2
Make FullPivHouseholderQR::solve returns the least-square solution instead of aborting if no exact solution exist
(grafted from 6814a13155ec120700c5b2b5d7e262f52323a43d)

--HG--
branch : 3.2
extra : source : 6814a13155ec120700c5b2b5d7e262f52323a43d