Determine function and necessity of header files for custom nodes

[matthewscharles]

This seems to be a requirement for registration -- it's proving difficult to stick to the format of the original tutorial at this point!

[matthewscharles]

This is all working now, albeit with some room for improvement. For now, I can comfortably create new nodes and build successfully, so I'm happy!

[matthewscharles]

Some references for the future:

• https://dev.epicgames.com/community/snippets/G1j/template-for-creating-metasound-nodes-in-c
• https://stackoverflow.com/questions/333889/why-have-header-files-and-cpp-files
• https://youtube.com/watch?v=OLQmeSsa9a0
• https://dev.epicgames.com/documentation/en-us/unreal-engine/epic-cplusplus-coding-standard-for-unreal-engine#physicaldependencies
• https://dev.epicgames.com/documentation/en-us/unreal-engine/epic-cplusplus-coding-standard-for-unreal-engine#generalstyleissues