[material,naviersstokes]

- added "constant" component which reads its properties from the input file
- updated documention for the "unit" component
- updated test_navierstokes which now uses the "constant" component

reviewed by bernd


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15395 2fb0f335-1f38-0410-981e-8018bf24f1b0