Automate diagram generation for manual

[jenetics]

The generation of the diagrams for the user manual requires a lot of manual work: trigger the generation and copying the graphs into the right directory. This generation process needs to be unified and automated. Ideally one script call, e.g. generate_graphs.sh, should trigger the generation process and copy the results into the correct directory.

The place where the diagram generation code is stored is spread across the modules. Find one central place for the diagram generation code.