[gnuplotinterface]

Added files for plotting the material laws. Added functionality to plot
files and functions in gnuplot, too. Two different examples how to use
the interface are added.
1) plot material laws like, pc-Sw, kr-Sw, D_eff-Sw, \lambda-Sw, etc:
   see test/implicit/2p2c/test/implicit/2p2c/waterairspatialparams.hh
2) live output of results:
   see test/multidomain/2cnistokes2p2cni/2p2cnisubproblem.hh

Test for the material plot functions will be introduced after the
release.

reviewed by gruenich




git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14458 2fb0f335-1f38-0410-981e-8018bf24f1b0