Define usage of Shape Path for Data Grant with InheritInstances scope

#130 uses viaPredicate to express predicate which directly relates parent to a child, we use it for inherited access.
I will link this issue inline in the primer to track changing it to st:viaShapePath.

Currently to find all Data Instances of child resources, we do following:

0. Each Data Grant is associated with specific ShapeTree (eg. Project, Task etc.)
1. Starting from child Data Grant with InheritInstances scope (eg. Tasks), follow interop:inheritsFromGrant #124
2. We get parent Data Grant with scope AllInsances or SelectedInstances (eg. Projects)
3. For each instance of parent (eg. Project)
4. Find and fetch ReferencesList for children matching ShapeTree of child Data Grant (there will be separate ReferencesList for each referenced child ShapeTree in parent ShapeTree).
5. In the ReferencesList find references to all child Data Instances

I think in 3.1 we should be able to do it purely based on child ShapeTree if we include that information in ReferencesList description (partially embedded in each parent Data Instance). In 3.2 we would need to use Shape Path to get references of children. We would also need to consider how based on that Shape Path relevant statements get added to ReferencesList.

@ericprud @justinwb

Start by reading issue #130, issue #124, and the primer reference mentioned in the issue. Trace how viaPredicate, st:viaShapePath, InheritInstances, and ReferencesList are currently described, then establish the agreed Shape Path usage and update the relevant primer or specification text when the semantics are settled.