Add typed API to pkg/attribute

[wangkuiyi]

This PR addresses sql-machine-learning/playground#42 (comment)

[tonyyang-svail]

LGTM

[wangkuiyi]

Here I should make it

func(x interface{}) error {
    v, ok := x(int)
    if ! ok {
         return fmt.Errorf("Attribute %s is of type int, but the value %v cannot be casted into int", name, x)
    }
    return checker(v)
}