Merge plot_3d and pfss_3d into one function

[jgieseler]

plot_3d and pfss_3d do more or less the same job. The main difference is that pfss_3d in addition plots some PFSS elements, and that the plot (and some of the calculations) is scaled to the solar radius, while plot_3d is scaled to astronomical units. Merging the two functions into one would simplify all kind of changes that need to be done to the functions.