Update to coding style

Until now we were using

clang-format-3.4 -style=Google -i file.c

as our default style. clang-3.4 is not supported any more on my machine (Ubuntu 16)... The current version is 3.8. Could we upgrade our style to 3.8 or at least to 3.5 (the oldest version available on my platform) ?

Note that cosma only offers clang 3.3 or 3.5...

That will change most of the files in the code though. What do you think ?