Restructured project file contents to better organize project directory.

[Alex-Vasile]

No functionality has been changed.

All I did was split the current files across several (one long align.swift and align.metal) was getting a little out of hand, especially for adding new functionality.

I also added a bit more documentation to a few functions, but there is a lot more to do.

[martin-marek]

@chris-rank you are currently much more familiar with the codebase than me (it might be mostly your code at this point). Are you happy with these changes?

[Alex-Vasile]

Instead of one large .swift and .metal file I've split it into:

• align: All the alignment related code
• frequency: All the frequency merging related code
• spatial: All the spatial merging related code
• exposure: All the exposure correction code
• texture: Helper functions for creating/manipulating Metal textures
• denoise: The main swift file that holds the main functions as well as the simple temporal averaging (wasn't sure if it made sense to create a whole new file just for it).
• constant.h: Some math constants used by the .metal files