Validate models against GO-CAM spec in ShEx

[dustine32]

Add step at end of model generation to validate model against the GO-CAM spec here in ShEx format:
https://raw.githubusercontent.com/geneontology/go-shapes/master/shapes/go-cam-shapes.shex

Would be cool if I could integrate some python ShEx parser like PyShEx to look directly at RdfLib graph object in memory. This way I can immediately respond to errors before writing out file. PyShEx has a CLI we can fallback on (checking the written out .ttl file) if using as a library doesn't work out.

First step of this ticket is to get some sort of feedback on models coming out. Then we'll look at some common errors/issues.