[freeflow]

- defaulted superlu and umfpack as linear solver
- added macros to skip test if the two above mentioned solvers are not
  installed
- removed duplicate property definition
- beautified comments

reviewed by gruenich and hommel


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15051 2fb0f335-1f38-0410-981e-8018bf24f1b0