Check if Enactment exists at given cite and date

[mscarey]

This validates that an Enactment really exists in a Code, kind of like how Enactment.text currently checks the Code before verifying that the text exists. Putting the method on Code instead should help eliminate Code as a param of Enactment (#25).

Both the cite and date for this check could come from the source path string linked to the Enactment class. Check the USLM schema for how to incorporate a date.

[mscarey]

Code class no longer exists. But maybe this goal could be accomplished by a Client method that checks for specified text, and then returns either an Enactment representing that text or an object representing the text's absence. See #22.

The "absence" could be limited to a particular statute range and date span.