Run IWYU on the opencs

The goal of this merge-request is to remove all redundant includes to reduce compilation time, and to do this we need to add all missing for each translation unit requiring them. The amount of #include lines will increase but total size of preprocessed code will decrease and !2450 (merged), !2465 (merged) actually did that.

The long-term goal is to add include-what-you-use in our continuous integration.