Fix trait bound on arr1 (op) &arr2

[jturner314]

The implementation of arr1 (op) &arr2 mutates arr1 for efficiency, but this caused surprising behavior if arr1 was ArrayViewMut. This commit requires that arr own its data to avoid surprising side effects.

This is an example of the old behavior that was surprising:

extern crate ndarray;

fn main() {
    let mut a = array![1., 2.];
    let b = array![3., 4.];

    // Prints a = [1, 2].
    println!("a = {}", a);

    {
        let a_view_mut = a.view_mut();
        // Prints a + b = [4, 6].
        println!("a + b = {}", a_view_mut + &b);
    }

    // Prints a = [4, 6]; would expect a = [1, 2].
    println!("a = {}", a);
}

Note that while this is a breaking change, the stricter trait bound is actually the correct bound according to the documentation.

[bluss]

Right, we have probably fixed this already for the other ops.

[bluss]

Thanks

[bluss]

Previous pr #93

Unrelated pr, but something we talked about today was #103