[yt-dev] Refactor of FITS frontend pull request

[John ZuHone]

Hi folks,

I have an open PR to refactor the FITS frontend:

https://github.com/yt-project/yt/pull/1576

This results in considerable simplification of the code in terms of readability and understanding what the heck it is doing. These changes will pave the way for the first test-bed for non-spatial coordinate systems. 

If someone else can have a look, I’d appreciate it--it’s ready to go. 

Best,

John