Update specification/stdlib_defs for new nodes

[dbsmythe]

Refactor procedural nodes into procedural, noise and shape nodegroups. Add descriptions of new shape nodes to specification. Add two colorspaces requested by Pixar USD team.

[dbsmythe]

While I was adding these documentation changes, I had a few questions that should be discussed and resolved:

• randomfloat/randomcolor: why do these have an "in" input? They also have a "seed", so what does "in" do?
• What does the "center" input for do?
• Should the default value for "center" in , and be (0.5, 0.5) and not the currrent (0,0)?
• I would assume that produces white gridlines on a black background, is that correct?
• Description of the "staggered" input for ?

[kwokcb]

FYI: As mentioned in the TSC sync :

The nodegroup meta-data tag reliance can be seen herefor instance in code generation, and would hence "break" if procedural2d and procedural3d were changed. I'm mention this in the PR as well.

There a check for geometry stream identification as well which is also "brittle".

[jstone-lucasfilm]

@dbsmythe Now that we've shifted our focus to the dev_1.39 branch for development of MaterialX 1.39, would you mind retargeting this pull request to that branch, and we can prepare it to be merged there?

[jstone-lucasfilm]

@dbsmythe Just bumping this thread, as it would be great to integrate these improvements into the dev_1.39 branch, making them part of the 1.39 release this summer.