New modeling has some entities resulting in strings instead of URIs

[bherr2]

Recently I've come across a couple of instances where a Subject or Object is a string instead of URI which breaks certain queries. Spatial Placement.placement_for does this, but there are likely others. Review all modeled data and determine which properties are not being parsed correctly to ensure they are processed as URIs instead of strings.

[bherr2]

This query: https://github.com/hubmapconsortium/ccf-grlc/blob/main/mesh-collision/anatomical-structures.rq broke because ccf:placement_relative_to started generating strings instead of URIs.