[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
This is weird that `&Q[0, 0]` with pointer arithmetic does not work.
Have you tried to define the pointer once and for all at the beginning of the function?

```python
cdef double* Q_ptr = &Q[0, 0]
```