C restructure

[daviesje]

A draft pull request for a restructure of the C backend. #359

Changes:

• We no longer include all headers and code in one file GenerateICs.c
• Instead, each file includes only the headers it uses, and all #include "X.c" have been removed
• build.cffi has been changed to make it clear which functions and definitions are available to the python wrapper.
  • CFFI requires all functions, global variables, and type definitions directly used in python to be present in cdef() calls as well as in the set_source() header files.
  • The cdef() code cannot contain compiler directives, as a result new header files were created labelled _something_wrapper.h to contain exposed definitions. these are also included in the relevant header files for the backend so we only have to edit one file to expose a function properly.
• C files UsefulFunctions.c and ps.c have been split up into functions grouped together by their themes e.g cosmology.c thermochem.c hmf.c etc...
• Some unused functions have been removed

Todos:

• Make directories for different categories of files, perhaps one for the output structures and one for lower-level functions?
• Remove the many unused variables declared throughout the code
• Stricter typing:
  • Indices for the entire box should already be unsigned long long, but single axis indices are sometimes int or unsigned long long.
  • Many lower-level functions still use float, most of these should be double unless they are a grid which takes up significant memory/disk space Migration toward double precision floating point numbers #361
• Think about splitting up heating_helper_progs.c as well
• Look into another build system i.e meson
• Set up C linting [Feature Req.] Automatic C linting #23

[steven-murray]

Couple of comments but overall this is SUPER nice. Thank you!!!

[steven-murray]

This is not your task necessarily (certainly not in this PR) but I'd love to have a nicer lower level wrapper that just exposes these functions directly in Python style, so you could call like

broadcast_structs(up, cp, ap, fo)
```

[daviesje]

do you mean just adding something like the following to the wrapper? these functions are already exposed

def broadcast_inputstructs(up,cp,ap=None,fo=None):
    if not isinstance(up,UserParams) or not isinstance(cp,CosmoParams):
        raise ValueError
    if isinstance(ap,AstroParams) and isinstance(fo,FlagOptions):
        lib.Broadcast_struct_global_all(up(), cp(), ap(), fo())

    lib.Broadcast_struct_global_noastro(up(), cp())

[steven-murray]

Exactly.