Improvements to StandardNodes tables

[jstone-lucasfilm]

Building upon the work in the new comparenodedefs.py script, this changelist makes several improvements to the tables in the StandardNodes specification, bringing the specification into closer alignment with the data libraries.

Using the output of comparenodedefs.py as a measure, this changelist reduces the number of differences between the StandardNodes specification and data libraries from 122 to 46.

Additionally, this changelist enables GitHub CI for changelists containing only edits to Markdown files, so that comparenodedefs.py is run on changes that contain only specification updates.

[jstone-lucasfilm]

For reference, here is the new output of comparenodedefs.py, when run on the specification and data libraries in this changelist:

Comparing:
  Specification: /home/runner/work/MaterialX/MaterialX/documents/Specification/MaterialX.StandardNodes.md
  Data Library: /home/runner/work/MaterialX/MaterialX/libraries/stdlib/stdlib_defs.mtlx

Parsing specification...
  Found 140 nodes with 659 node signatures
Loading data library...
  Found 143 nodes with 689 node signatures

Comparing node signatures...

======================================================================
COMPARISON RESULTS: 46 difference(s) found
======================================================================

Nodes in Specification but not Data Library (1):
--------------------------------------------------
  displacement

Nodes in Data Library but not Specification (4):
--------------------------------------------------
  ramp
  ramp_gradient
  surfacematerial
  volumematerial

Nodes with Different Input Sets (13):
--------------------------------------------------
  dot: (in:boolean) -> out:boolean
    Extra in library: note:string
  dot: (in:color3) -> out:color3
    Extra in library: note:string
  dot: (in:color4) -> out:color4
    Extra in library: note:string
  dot: (in:filename) -> out:filename
    Extra in library: note:string
  dot: (in:float) -> out:float
    Extra in library: note:string
  dot: (in:integer) -> out:integer
    Extra in library: note:string
  dot: (in:matrix33) -> out:matrix33
    Extra in library: note:string
  dot: (in:matrix44) -> out:matrix44
    Extra in library: note:string
  dot: (in:string) -> out:string
    Extra in library: note:string
  dot: (in:vector2) -> out:vector2
    Extra in library: note:string
  dot: (in:vector3) -> out:vector3
    Extra in library: note:string
  dot: (in:vector4) -> out:vector4
    Extra in library: note:string
  time: () -> out:float
    Extra in library: fps:float

Node Signatures in Data Library but not Specification (28):
--------------------------------------------------
  convert: (in:boolean) -> out:surfaceshader
  convert: (in:color3) -> out:surfaceshader
  convert: (in:color4) -> out:surfaceshader
  convert: (in:float) -> out:surfaceshader
  convert: (in:integer) -> out:surfaceshader
  convert: (in:vector2) -> out:surfaceshader
  convert: (in:vector3) -> out:surfaceshader
  convert: (in:vector4) -> out:surfaceshader
  dot: (in:displacementshader, note:string) -> out:displacementshader
  dot: (in:lightshader, note:string) -> out:lightshader
  dot: (in:surfaceshader, note:string) -> out:surfaceshader
  dot: (in:volumeshader, note:string) -> out:volumeshader
  fractal2d: (amplitude:float, octaves:integer, lacunarity:float, diminish:float, texcoord:vector2) -> out:color3
  fractal2d: (amplitude:float, octaves:integer, lacunarity:float, diminish:float, texcoord:vector2) -> out:color4
  fractal2d: (amplitude:vector3, octaves:integer, lacunarity:float, diminish:float, texcoord:vector2) -> out:color3
  fractal2d: (amplitude:vector4, octaves:integer, lacunarity:float, diminish:float, texcoord:vector2) -> out:color4
  fractal3d: (amplitude:float, octaves:integer, lacunarity:float, diminish:float, position:vector3) -> out:color3
  fractal3d: (amplitude:float, octaves:integer, lacunarity:float, diminish:float, position:vector3) -> out:color4
  fractal3d: (amplitude:vector3, octaves:integer, lacunarity:float, diminish:float, position:vector3) -> out:color3
  fractal3d: (amplitude:vector4, octaves:integer, lacunarity:float, diminish:float, position:vector3) -> out:color4
  noise2d: (amplitude:float, pivot:float, texcoord:vector2) -> out:color3
  noise2d: (amplitude:float, pivot:float, texcoord:vector2) -> out:color4
  noise2d: (amplitude:vector3, pivot:float, texcoord:vector2) -> out:color3
  noise2d: (amplitude:vector4, pivot:float, texcoord:vector2) -> out:color4
  noise3d: (amplitude:float, pivot:float, position:vector3) -> out:color3
  noise3d: (amplitude:float, pivot:float, position:vector3) -> out:color4
  noise3d: (amplitude:vector3, pivot:float, position:vector3) -> out:color3
  noise3d: (amplitude:vector4, pivot:float, position:vector3) -> out:color4

[niklasharrysson]

Nitpick: For consistency with the other nodes would it make sense to have the output define the data type, and let default derive from this? That is, just flip the relation here so default says Same as 'out'

[jstone-lucasfilm]

Good idea, and I've made this change across the texture nodes for consistency.

[niklasharrysson]

This looks good to me, and I only had a minor comment above.