public static SimpleImputer fit(DataFrame data, String... columns) {
        return fit(data, 0.5, 0.5, columns);
    }
@Test
    public void testUSArrests() throws Exception {
        System.out.println(USArrests.data);
        SimpleImputer imputer = SimpleImputer.fit(USArrests.data);
        System.out.println(imputer);
    }