Basic 3D plots, bug fixes, possibility to superimpose gramm plots

- ‘z’ input data in gramm() creates 3D plots when using geom_point() an
geom_line()
- Corrected bug on boxplot where no boxes were drawn if there was no
color input
- Corrected whisker length (was indicating outlier limit instead of
last points that are not outliers)
- Added possibility to superimpose gramm plots
- Added support for categorical data