With the ability to embed pipelines/graphs within composition,
the use of the term "solid" through the process becomes more
This renames ISolidDefinition to NodeDefinition to indicate something
that can embedded in a graph (which can be a graph or a solid) and
begins to rename the internal implementations to be "nodes" instead of
"solids". Otherwise variables with the name "solid" could actually
refer to pipelines/graph.
Depends on D4888