message: Change glTF PBR version from 0.0.1 to 2.0.1
  Since the glTF PBR is pretty much complete in its current state and is publicly used, it seems appropriate to change its version number to match the glTF specification (2.0.1).  For comparison: UsdPreviewSurface is at 2.3 and Standard Surface at 1.0.1.
diff: <?xml version="1.0"?>
  <materialx version="1.38">
  - <nodedef name="ND_gltf_pbr_surfaceshader" node="gltf_pbr" nodegroup="pbr" doc="glTF PBR" version="0.0.1" isdefaultversion="true">
  + <nodedef name="ND_gltf_pbr_surfaceshader" node="gltf_pbr" nodegroup="pbr" doc="glTF PBR" version="2.0.1" isdefaultversion="true">
  <input name="base_color" type="color3" value="1, 1, 1" uimin="0, 0, 0" uimax="1, 1, 1" uiname="Base Color" uifolder="Base" />
  <input name="metallic" type="float" value="1" uimin="0" uimax="1" uiname="Metallic" uifolder="Base" />
  <input name="roughness" type="float" value="1" uimin="0" uimax="1" uiname="Roughness" uifolder="Base" />
  </nodedef>
  <nodegraph name="IMPL_gltf_pbr_surfaceshader" nodedef="ND_gltf_pbr_surfaceshader">
  +
  <!-- Volume -->
  <convert name="attenuation_color_vec" type="vector3">
  Node: <gltf_colorimage> Supplemental Node
  Multiply a color4 image modulated by uniform color and geometry color. Default uniform and geometry colors are 1,1,1,1 which results in no change to the image.
  -->
  - <nodedef name="ND_gltf_colorimage" node="gltf_colorimage" version="0.0.1" isdefaultversion="true" nodegroup="texture2d">
  + <nodedef name="ND_gltf_colorimage" node="gltf_colorimage" version="1.0" isdefaultversion="true" nodegroup="texture2d">
  <input name="file" type="filename" uniform="true" value="" uifolder="Image" />
  <input name="default" type="color4" value="0, 0, 0, 0" uifolder="Image" />
  <input name="uvindex" type="integer" value="0" uifolder="Image" />
  