Function efd::diff

pub fn diff<'a, A, D, V>(arr: V, axis: Option<Axis>) -> Array<A, D> where
    A: NumOps + ScalarOperand,
    D: Dimension,
    V: AsArray<'a, A, D>, 

Calculate the n-th discrete difference along the given axis. Same as NumPy version.

Reference from https://github.com/rust-ndarray/ndarray/issues/787.