Use `_grid` instead of `_native_backend` in the Backend

[BDonnot]

Though (again) it's not written in the doc (my mistake again) it would be better and more consistent with the grid2op ecosystem to rename the private attribute self._native_backend to self._grid.

I can do the PR if you prefer of course.

[BDonnot]

Right now, _grid points to a Backend, but in grid2op philosophy, backend._grid should be a 'pp.network.Network'