Better shape validation

[landonreed]

Recently some shapes created in the Data Tools editor were discarded during graph building by OTP. To give Data Tools users a sense for when this might happen to their GTFS feeds, I think we should add some functionality similar to the GTFSPatternHopFactory#isValid method.

cc @abyrd

[abyrd]

Should the editor even be capable of creating invalid shapes? What was wrong with the shapes it produced which caused them to be rejected by OTP?

I agree we want to sanity check inputs an outputs as much as possible. My main concern would be maintaining duplicate code - is it possible to design these checks such that they can be reused in GTFS-lib and other projects?