added possibility to pass user-defined options (in gnuplot language),
datafile separator can now be chosen, and always a gnuplot script will be
produced

[plotoverline2d.hh]
added switch to rewrite instead of append to data file, removed
superfluous tabs

[evaporationatmosphereproblem.hh]
added example for use of gnuplotinterface and plotoverline2d

reviewed by gruenich



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14876 2fb0f335-1f38-0410-981e-8018bf24f1b0