Header files in a different directory

[jordanflitter]

Right now the header files of v4 are at the same directory of the C files, making the src directory a bit hard to navigate as it currently contains 64 files. Alternatively, we can do something similar as in CLASS, and put all the header files in an "include" directory (see https://github.com/lesgourg/class_public/tree/master/include vs. https://github.com/lesgourg/class_public/tree/master/source).

[daviesje]

I'm sure there's a lot of debate and personal preference about the organisation of packages, and while I'm open to suggestions, I prefer the layouts where the .c/.h pairs are kept together, and subdirectories are made based on the purpose of the files. We should definitely talk about the folder layout before the release of v4 though.