Merge branch 'cleanup/typos' into 'master'

[cleanup] fix some typos in documentation strings

I still had some old commits lying around that fixed some typos in comments. Since then I found the codespell program which turned out to be quite helpful in finding common misspellings.

So I guess, I've gathered enough fixes by now to justify a merge request. 

Similar merge requests for the rest of the Dune core modules will follow soon.

See merge request !79