Linalg update

linal_update is simply a rordering of the commits in symeig_update  perhaps much uptodate to master.
You can merge it with master (or merge symeig_update if you prefer, but only one of the two)

after that there will still be some cleaning to do an also file reorganization in linalg (I had some talks with joel)
changing directory tree an passing some lapack partial interfacing functors to details namespace.

I think that an extern lapack module will be great as soon as we pass to C++1y but not before as many lapack
functions will lead to too many parameters in nt2 functors.