bugfixes and extensions for free_surface support

See merge request !3

Minor updates to ExprTk

See merge request !5

Updated multi-sub expression operator to return final sub-expression type.
Updates to exprtk_disable_return_statement macro for disabling return statements and associated exceptions at the source code level.
Various other updates

Delete CTestConfig.cmake

See merge request !2

If you have access to the old closed source waLBerla repository, you may restore the complete history with the following command
git fetch git@i10git.cs.fau.de:software/walberla.git 'refs/replace/*:refs/replace/*'