Adds coding guidelines to doxygen.
For Review outlook of the final result in doxygen: https://soe.peters.gitlab-pages.rz.tu-bs.de/VirtualFluids/coding-guidelines.html
Edited by Sören Peters
For Review outlook of the final result in doxygen: https://soe.peters.gitlab-pages.rz.tu-bs.de/VirtualFluids/coding-guidelines.html