Function sum

src/danfojs-node/test/core/frame.test.ts:1559–1561

(x: any)

                const df = new DataFrame(data)

                const sum = (x: any) => {
                    return x.reduce((a: any, b: any) => a + b, 0)
                }
                const expected = [6, 7, 3]
                const dfApply = df.apply(sum, { axis: 1 })
                assert.deepEqual(dfApply.values, expected)