[doc] fix as much typos as possible unto next release
Description
when one reads through the DuMux core codes comments and other documentation, one can find sometimes comments that are obviously not correctly describing the underlining code, or there are typos. It would be worth to try keeping this as tidy as we could as typos and incorrect comments are limiting the readability.
It is worth the effort, to fix as much obvious (and possbibly also some less obvious) typos or inconsistencies until the upcoming release.
Branch, that focuses on the freeflow
part: cleanup/smallCleanups. The changes are currently all limited to freeflow
Branch focusing on fluidsystems: cleanup/fluidsAndFluidsystems
Edited by Kai Wendel