[Task] TinyUSDZ method API naming rule

[syoyo]

Currently TinyUSDZ does not have unified, consistent API naming rule for methods. Some uses GetValue, some uses get_value.

pxrUSD uses Upper Camel case C#-like naming rule(e.g. GetValue). https://en.wikipedia.org/wiki/Camel_case

TinyUSDZ does not try to implement all of pxrUSD features. To avoid confusion, it'd be better to use different API naming rule from pxrUSD, so TinyUSDZ will use Snake case https://en.wikipedia.org/wiki/Snake_case , which is much common in C++, Rust and Python. Rewriting API naming rule may take some time.

pxrUSD compat API may be provided in different layer: https://github.com/syoyo/tinyusdz/blob/dev/src/pxr-compat.hh