ExecUsd: Execution system for Usd

The execUsd library is built on top of exec and esfUsd.

The execUsd library is the primary entry point for OpenExec. The API defined here, and in the exec library, supports:

• Registration of computational behaviors associated with USD schemas
• Ingesting a UsdStage to compile the data flow network that contains nodes that embody computations
• Requesting values for efficient, vectorized, multithreaded evaluation

See the OpenExec overview for more details.