Adding LamaSurface Node

[mkuo-lucasfilm]

Adding initial support for LamaSurface node (pixar renderman definition: https://rmanwiki.pixar.com/display/REN26/LamaSurface)

• LamaSurface has two inputs of BSDF type and one input of float type, and returns a surfacematerial
• LamaSurface node example is commented out, as there's an issue with the shader generation process, which doesn't yet support the notion of a custom node that returns a surfacematerial

[jstone-lucasfilm]

This looks good to me, thanks @mkuo-lucasfilm!