ppafm-plot-results --atoms requires input_plot.xyz

[mondracek]

When I call the plot_results.py script with the --atoms option to make it show atom positions in the PNG images it creates, the script requires input_plot.xyz file to be present, otherwise ends with an error. While the file input_plot.xyz might be okay as the default name for the input file with atom coordinates, I think the script should also accept the -i parameter for the input file name and various possible input file formats besides the XYZ one, just as the other scripts do.