Enable subdiv by default when set in the UsdGeomMesh schema

Currently, even when the usd mesh attribute subdivisionScheme is set e.g. to catclark (which is the default), there is no subdivision in Arnold because we force the default value of subdiv_iterations to be equal to 0.
Therefore, there is no way to enable subdivision without setting the arnold specific attribute.
We should consider changing this and let arnold have a default subdiv iteration of 1

Trace how UsdGeomMesh subdivisionScheme and Arnold's subdiv_iterations are handled, starting with the mesh translation path. Verify the default catclark case and determine whether one subdivision iteration is applied without an Arnold-specific attribute; done means the behavior is covered by the relevant tests.

Written by the indexing model from the issue text.