[ci] Enable codespell spell checking

Adds a job that checks spelling in Dumux code

• Run codespell -w to actually implement the changes before enabling this check
• Go through the automatic corrections and fix all false positives

Notes to reviewer:

• Changes some internal variable names insideD and aws to better names insideDiffCoeff areaWS
• Fixes one spelling mistake in an interface DiffusionCoefficientAveragingType in darcy-stokes Arithmethic -> Arithmetic. If someone used the wrong spelling before this will lead to an error now. There is a note about it in the changelog.
• Some spelling mistakes in tex files are fixed but tex-files are excluded in the CI because there were some false positives that are hard to fix

Closes #1174 (closed)