Empty plot for stage5 `annotations` script

[cosimolupo]

The script annotations - present in both versions of stage5 - contains some lines of code for saving a plot:

    # ToDo
    if args.output_img is not None:
        save_plot(args.output_img)

In fact, as highlighted by the "ToDo" string, it is an empty plot, and it is saved as such. Before that, a .csv is however created, and correctly filled and saved. Since this block is meant for the creation of such annotation file, my guess is that a plot is not necessary, and hence those lines should be removed, to avoid users to be left with an empty canvas.

[cosimolupo]

Snakemake (through compute_measure template rules) requires for each measure two outputs: a .csv file, and a plot. Removing such lines at the end of the annotations block would cause a Snakemake failure, then.
This solution would in fact imply a refactoring of the stage5 Snakefile, possibly distinguishing between blocks producing plots, and blocks that do not.
If annotations is the unique block not producing a block, then a lighter solution is to execute it via an explicit annotations rule, using the compute_measure template rule only for other blocks producing plots.