Remove myself as maintainer

Happy 5th birthday to Allpix Squared! 🎉 . Today marks the day that it is exactly 5 years ago since v1.0 was released. Time flies since my time as technical student at CERN...

Success is always hard to quantify, but it goes without saying that this is among the most successful pieces of software that I co-created. Its been a long journey since the first release, with many new versions after that, including as highlight of course the much optimized v2.0 (which was a lot of fun to contribute too!).

Even after 5 years the project is still running strong. Of course a lot of other improvements have been made that would not have been possible without the help of many contributors, but in particular thanks to @simonspa and @pschutze. I don't think I am really worth to be called a maintainer anymore among you two ;).

To conclude I thought this 5 years mark gave a nice opportunity to 'officially' remove myself as a maintainer, and join the 'contributors'. Also removed myself from the maintainers in the modules, and replaced with Simon where it became empty (of course let me know if I should add someone else).

By the way It doesn't mean I am gone forever now. I might still contribute something every now and then, we'll see! Hope there will be many years of AP2 to come 😄 .

Cheers, Koen