Add experimental SheX for a GO-CAM limited to the GPAD spec. (#307)

* Add experimental SheX for a GO-CAM limited to the GPAD spec.

* Add more comments.

* Change Enabler to DBObject.

* Add 'has primary input/output' relations to allowed GPAD extensions.

* Add some incoming constraints.

* Add GP-to-BP relations, and handle negated GO terms with extensions.

* Add 'existence starts and ends during'.

* Relax string literal constraints.

* Make class declarations optional.

* Add 'regulates transport of' to extensions.

Author: balhoff

shapes/gpad-shapes.shapeMap

@@ -0,0 +1 @@
+{ FOCUS <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://www.w3.org/2002/07/owl#NamedIndividual> }@<http://purl.obolibrary.org/obo/go/shapes/Individual>