Refer to DG node by name, iterate over shadingGrou

[brunobowden]

1) Firstly, is there a way to refer to a DG node, not a DAG node, by name? Similar to MDagPath. For example to get the MObject for "renderPartition".

2) Secondly, what about iterating over DG nodes, but again not DAG nodes. For example to process all the shading groups in turn.

One way I could do this is to access the renderPartition node and use MItDependencyGraph to go upstream to all the shading groups.