[WIP] Releasing the GIL in the inner loop of coordinate descent

- [x] release the GIL in the dual gap check (need to replace numpy calls by BLAS equivalents)
- [x] release the GIL in the sparse input variant
- [x] release the GIL for the precomputed Gram matrix variant
- [ ] benchmark the scaling for instance by hacking the cross_val_score function to make it possible to use the threading backend instead of multiprocessing
@ogrisel Is there are a way of accessing the row-wise address? i.e to do `&Q[ii, 0]` just using the address of the first element, something like, `*(Q + ii)` in C++ . This would make it independent of whether it is C or Fortran contiguous.