Draft: cleanup/smallCleanups

As mentioned in 1346 several things should be improved in the documentation. In this branch, a large part is concerned with fixing spelling mistakes. Besides this, some obviously useless code parts were removed. Some TODO-comments were removed, as it seems like that the task referred to is done, but I was not sure about it. Also in one test (channel flow) an additional comment was added, which could be useful (see commit 05b5753a

This merge request is also declared as draft as I would expect several commits could cause some discussions, as the proposed changes may bring some unexpected implications.

Note also, that the changes are nearly all limited to the freeflow part of DuMux.