[cleanup] Whitespace cleanup in dumux/material.

- remove trailing whitespace
- replace tabs by spaces
- remove pointless returns
- remove superfluous semicolons
(reviewed by fetzer)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15056 2fb0f335-1f38-0410-981e-8018bf24f1b0