Recursively matching and getting subordinary models within the compos…

[lhy11009]

…iting material model

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I added a recursive utility of matching and getting subordinary models within the compositing material model.

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[lhy11009]

@naliboff Please take a look. I tried to achieve minimal change to the code in the compositing.h and create these two separate functions based on their origin in the Plugins class. Ideally, this could be reused in places where we need to specifically access the material model plugins.