Bug fix: Return zero if there is no directed path from treatment to outcome

[amit-sharma]

• Added a check for directed path from treatment to outcome
• If no such path exists, estimator is not called. Effect is zero by definition.

[pawni]

I just checked that this implementation fixes the previous bug.

However, should methods like CausalEstimate.get_standard_error handle the missing estimator or should this be handled downstream by the user?

[amit-sharma]

Handling downstream by user should be fine for now. Since the standard error will tell the user "self.estimator not found", which is essentially the same exception we would raise if we handle it in get_standard_error.