Improve to_ndarray with better dimensionality awareness

[Enet4]

At the end of #1, there was a discussion of whether it would be possible to specify the dimensionality for the Array created with to_ndarray. In particular, it might be more useful to have this method prototype instead:

fn to_ndarray<T, D>(self) -> Result<Array<T, D>>
where
    T: ...,
    D: IntoDimension;

I will leave this issue for discussion on the feasibility of this. Basically, this requires a conversion (which can fail) from a dynamic shape to an arbitrary dimension type D. I haven't found a way to do this yet in ndarray. There is also the small drawback that one has to specify two parameter types instead of just one. IxDyn is a good default, but default type parameters do not work on functions (rust/#36887).

[Enet4]

It turns out that there is .into_dimensionality(), which would make it significantly easier to obtain a fixed dimension array.

let arr = volume.to_ndarray()?.into_dimensionality::<Ix3>()?;

I would suggest leaving to_ndarray alone and advise users to rely on this method if so is desired.