Fixed a bug related improperly passing through parse_symtab call when extended USR is called recursively.

Fixed problem related to case-sensitivity of locally defined variables
Fixed problem related to compilation with latest version of clang (version 5.0.0 trunk 303036)
Minor code cleanup

Added compile-time check for vector indexing when using constant values

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

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/*'