Describe the feature you'd like to see

Snakemake provides a message attribute which can be used in rules to provide a user-friendly read-out to the console, including wildcards

rule NAME:
    input: "path/to/inputfile", "path/to/other/inputfile"
    output: "path/to/outputfile", "path/to/another/outputfile"
    threads: 8
    message: "Executing somecommand with {threads} threads on the following files {input}."
    shell: "somecommand --threads {threads} {input} {output}"

PyPSA-Eur has around 60 rules, and only three or four of them use messages.

Messages provide a useful way of documenting the purpose of rules, and explaining at run-time what is going on. I think adding messages to each rule would help new users of PyPSA-Eur, and also aid with debugging the snakemake workflow.

Pull request open-energy-transition#55 already includes many of the rule messages needed, so implementing this is just a matter of copy-pasting the relevant lines into the snakemake rules.