start dftplots examples

[johuck]

this pull request adds a dftplots subdirectory including README

[johuck]

The script gp_bindenergy is based on gp_datdir and works now with my data input. The input data is similarly organized to the examples dir. The gp_bindenergy dir in data consists of subdirs corresponding to the calculated guest molecules with the resulting data for every tested functional as saved .txt.
Next, the circle plot shall be replaced by a bar plot comparing each metal and functional. The graph should in the end look similar to the second plot on the gnuplot demo site (however without the lines) with the metals distinguished on the x-axes and the functionals compared as different colored bars.