specular_weight reinterpretation (as decoupled IOR) (#247)

* Proposed changes to make specular_weight only affect reflection, not IOR

* Revert "Proposed changes to make specular_weight only affect reflection, not IOR"

This reverts commit dc13556.

* Add reflection/refraction decoupling details to spec

* Add figure. Add new Dielectric Base section.

* Add clamp back

* Fix angles in diagram

* Simplified implementation of "decoupled IOR" for MaterialX

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Co-authored-by: Adrien Herubel <AdrienHerubel@users.noreply.github.com>

Author: portsmouth