What this MR does / why does DuMux need it:
This MR updates the versions of Dumux and Dune in the handbook. Also, the images for tpfa, mpfa and box have been unified and the text for the box method has been updated.
Notes for the reviewer
You need to compile the pdf of the handbook locally.
Before you request a review from someone, make sure to revise the following points:
\n character. Make sure there is no
\ No newline at end of file comment in "Changes" of this MR.