IECoreCycles does not handle OpenUSD's constant `primvars:displayColor`

[andrewkaufman]

Version: Cortex 10.5.8.0 (as included with Gaffer 1.4.6.0)
3rd-party modules: Cycles (4.0.0 as included with Gaffer 1.4.6.0)

Description

When working on GafferHQ/gaffer#5926 I recieved the following terminal output:

WARNING : IECoreCycles::Renderer : Expected Color3fData but got Color3fVectorData for attribute "render:displayColor".

As cumbersome as it is, OpenUSD prefers vector/array data for a single constant color primvar 🤐

Steps to Reproduce

1. Follow the steps in the linked Gaffer bug & check your terminal