Method find

src/matrix.rs:699–706

TODO tests

(&self, f: F)

    // TODO tests
    pub fn find<F>(&self, f: F) -> Vec<(usize, usize)> 
        where F: Fn(&T) -> bool {

        self.values().enumerate()
            .filter(|&(_idx, val)| f(val))
            .map(|(idx, _val)| (idx % self.cols(), idx / self.cols()))
            .collect()
    }

    /// Inserts a column before the specified column (indexing starts at zero).
    ///