`NdArray` trait

[bluss]

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We need to traitify the array types, so that we shift from using ArrayBase for generic programming to something more flexible.

Imagine a sketch:

pub fn std_axis<T>(array: T) -> Array<T::Elem, T::Dim::Smaller>
    where T: NdArray,
          T::Elem: Num,
{
     // implementation here.
}

Foreseen problems: How do we conditionalize methods?

• Can mutate the axes or view
• Can mutate elements
• etc