Add C function to read token file

[LiamPattinson]

Adds the function fdp_read_token that gets the registry token from a file. It's a simple utility, but I had to implement it from scratch in the Fortran API, and exposing it in the C API means that any future changes to the C++/C methods will be automatically carried forward to Fortran or anything else that hooks into the C API.

Also applies clang-format to some older code -- not sure how this was missed the first time around.

[LiamPattinson]

For the failing tests:

• The C simple model was trying to use the branch c_api, which has since been merged. I've corrected this now.
• The code analysis is complaining about the use of a C function strncpy. It looks about as safe as I can make it.
• No idea why the Windows build is failing. I can't find anything in the log that indicates where the error is.