make a validate_features function

[rkurchin]

Copying over from #85 ...

I had to comment out some checks in the main AtomGraph constructor that were validating that the encoded features actually matched the featurization (because it now dispatches on AbstractFeaturization rather than GraphNodeFeaturization specifically), so we could just make a new function like validate_features(encoded, fzn) that would dispatch across different featurization types, and if you fed in a DummyFeaturization it would always say it was okay.

The abstract dispatch is actually already there in #87 so "all" that remains is actual implementations (envisionable, though certainly not entirely trivial).