New "development" category and some category reshuffling

Hi everyone,
I’ve added a new #development category, with a first post with request for feedback for a new feature: Running multiple standalone simulations [Request for Feedback]

I also used the opportunity to merge a few categories into groups, e.g. #science:projects and #science:showcase are now sub-categories of #science. This should not change much in practice (e.g. all links and cross-references between topics are unaffected), but I feel that it gives a somewhat clearer picture of the available categories.