Named Conversion in Houdini

This request stops the use of blindData names when converting VisibleRenderables to/from Houdini. It adds a name parameter for the ToHoudiniGeometryConverter, so names going into Houdini can be specified explicitly. The FromHoudiniGeometryConverter provides a utility function for extracting a named bit of Houdini geo from a GU_Detail. Processes interested in partial conversion of GU_Details have been updated to use this extraction before creating their converters.

This is the first of several pull requests which break binary compatibility for IECoreHoudini. This one also breaks python compatibility for the FromHoudiniGeometryConverter.create() factory function.