Add .clang-format file

This merge request adds a .clang-format file to Aevol.

A .clang-format file describes the rules we use for automatically formatting files.

The goal is to incrementally reformat files as we work on them, in order to improve code readability for everyone.

It also exports the build commands (the exact command executed to build each file) into a compile_commands.json file used by the C++ plugins of different IDEs.