Add monotonic check in FunctionFactory

[zhyass]

Summary

/// The property of monotonicity for a certain range.
struct Monotonic {
    // Is the function monotonous (nondecreasing or nonincreasing).
    is_monotonic: bool, 
    // true if the function is nondecreasing, false, if notincreasing. If is_monotonic = false, then it does not matter.
    is_positive: bool, 
    // Is true if function is monotonic on the whole input range
    is_always_monotonic: bool, 
}

[junli1026]

/assignme