Add BrepObject.GetSubObjects()

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RhinoCommon: https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_DocObjects_RhinoObject_GetSubObjects.htm

A BrepObject can be composed of several subobjects, each with their own material. BrepObject.GetSubObjects() returns the parts of each BrepObject, each with their attributes and material id.

(If there is an alternative way of getting the subobject attributes, then that would be ok as well)

[mcneel-build]

Linked with RH3DM-136

[fraguada]

DocObjects are not part of rhino3dm