Prefactoring: Rename and move interfaces

[jeremykubica]

This is an initial set of changes to prepare for the ImageStack redesign, including:

• Use a consistent name (obstime) throughout the stack for the time of observation.
• Move the storage of the observation time from LayeredImage to RawImage.
• Change PPI to NPixels throughout the stack.
• Move the majority of FITS file reading into RawImage, adding convenience functions for appending layers, reading in an image from a specific layer, and write a single layer file.
• Write out per-image timestamps (instead of a single one per LayeredImage)
• Add more tests for file io.

[DinoBektesevic]

It's a bit confused as to what naming convention we're using - but that's job for a different day.

Feels pretty good to finally sort these annoyances out.