Merge branch 'cleanup/3.7' into 'master'

[exercises] Updates/cleanup for 3.7

See merge request !143

(cherry picked from commit b4c16461)

9ae93f05 [exercises] upgrade license
922b3dc7 [cleanup] Update exercise basic
ede82630 [cleanup] Use constexpr in instead of enum
12856e89 [cleanup] Remove clutter
0da6c871 [cleanup] Fix spelling mistakes with codespell
b162c1af [cleanup] Use more static constexpr int instead of enum
4ee41735 [cleanup] Remove ctime and dumux message
0a08f5e3 [cleanup] Use new solvers, no setPreviousSolution, initialize
c2ad627a [cleanup] Replace enums with static constexpr
608e50a5 Update some code snippets in readme