Method get_mut

src/matrix.rs:563–569

TODO test

(&mut self, row: usize, col: usize)

    // TODO test
    pub fn get_mut(&mut self, row: usize, col: usize) -> Option<&mut T> {

        match self.idx(row, col) {
            None => None,
            Some(p) => self.data.get_mut(p)
        }
    }

    /// Returns the row at index `n` (starting at zero) in O(1).
    ///