Return numpy arrays instead of lists in Python interface

[alecandido]

At the moment, we're not consistently dealing with arrays/lists in python, since we only tried late the rust_numpy crate.

We should then move the formerly implemented methods that returns Vec<T> (handled in python as a list) to return whatever PyArray is suitable from the mentioned crate.

New with rust_numpy:

https://github.com/N3PDF/pineappl/blob/a1fe0350b00a1129e35d680c5dfa4448a59f8fa6/pineappl_py/src/grid.rs#L410

old with Vec<T>:

https://github.com/N3PDF/pineappl/blob/a1fe0350b00a1129e35d680c5dfa4448a59f8fa6/pineappl_py/src/grid.rs#L427

https://github.com/N3PDF/pineappl/blob/a1fe0350b00a1129e35d680c5dfa4448a59f8fa6/pineappl_py/src/grid.rs#L444

[cschwan]

Yes, that's a good point!