I implemented this function to expose `_factor_lehman_method()` at user level. If the user wants to use lehman method directly he/she can do it now. It didn't feel right to declare `_factor_lehman_method()` here as it is, since it is using `mpz_class` and  (If I am not mistaken) user has no way to use `mpz_class`. Also I didn't want to change the function signature of `_factor_lehman_method()` to use `Integer` type and pointers since other functions in `ntheory.cpp` might want to call it and it will be convenient for them to call `_factor_lehman_method()` as it is now. We can now test `_factor_lehman_method()` through this.