indentation: replace all tabulator characters by four spaces

This messes up the indentation in some places, but before it was
messed up anyway, depending on how your editor displays tabulator
characters.

To avoid such things in the future, please change your editor not to
insert 4 space character instead of one tabulator character.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6079 2fb0f335-1f38-0410-981e-8018bf24f1b0