Control naming of materials primitives in the writer

[sebastienblor]

Currently, the usd writer adds a "materials" prefix to the materials it authors. It could be useful to control this, or to set it otherwise so that we don't loose the hierarchy and prefixes from the scene. In particular, exporting from MtoA with a given prefix will appear in the node names, and the materials prefix will be placed before the MtoA prefix.

[autodesk-oss-arnold-bot[bot]]

Issue synced internally to ARNOLD-13290