Add try_into_owned_nocopy method

[jturner314]

I've been working on improvements to the layout handling in ndarray-linalg to make the code easier to understand, fix bugs, and avoid unnecessary cloning and by-clone transposes. One of operations I need to implement is, "Convert an ArrayBase into a LAPACK-compatible Array (i.e. strides = [1, s], with s >= nrows), while avoiding unnecessary cloning." I don't think it's possible to implement this today without adding more methods to ArrayBase. To address this, I propose adding a try_into_owned_nocopy method which is similar to into_owned but returns Err(self) if the data would need to be cloned, instead of actually performing the cloning. This way, if the data needs to be cloned, I can clone it into an array of the correct layout, like this:

match array.try_into_owned_nocopy() {
    Ok(o) if is_lapack_compatible(&o) => o,
    Ok(o) => clone_into_new_lapack_array(&o),
    Err(a) => clone_into_new_lapack_array(&a),
}

There are other ways this problem could be solved, but try_into_owned_nocopy is the best I came up with.

A couple of notes about the PR:

1. If we implement DataOwned for CowRepr in another PR, then I'd change the bound on try_into_owned_nocopy to be S: DataOwned, since there wouldn't be any reason to call it on any non-DataOwned types. Edit: I was wrong; the method is actually useful for S: Data after all, and there isn't a strong reason to restrict it to S: DataOwned anyway.

2. I'm not completely satisfied with the name try_into_owned_nocopy. Other suggestions would be welcome.

[bluss]

We want to merge this right? I think the name will be ok

[jturner314]

Yes, I still think this is a good idea, and I agree that the name is good enough.

[bluss]

Great, thanks!