Rename append_row/column to push_row/column; add method push

[bluss]

Follow up to #932 by renaming append methods for consistency, as outlined in #932 (comment) :

// On 2-D arrays
fn push_row(&mut self, row: ArrayView<A, D::Smaller>) {}
fn push_column(&mut self, column: ArrayView<A, D::Smaller>) {}

// On n-D arrays
fn push(&mut self, axis: Axis, array: ArrayView<A, D::Smaller>) {}
fn append(&mut self, axis: Axis, array: ArrayView<A, D>) {}

This means we have two similar methods - push and append.