Use structopt for CLI

[Razican]

structopt is a crate that allows generating command line interfaces that map to a struct. This would avoid this code in the main.rs file, and improve the validation:

    // Read command line arguments
    let args: Vec<String> = env::args().collect();
    if args.len() < 3 {
        eprintln!("xisfits [input file name].xifs [output file name].fits");
        process::exit(1);
    }
    println!("Args: {:?}", args);

    // Open XISF image file
    let xisf_filename = &args[1];
    let fits_filename = &args[2];

This could also be an opportunity to add a --verbose or -v mode, where all the println!() happens (Unix commands are supposed to not print anything if things go well). This would also help with the performance: writing in the terminal takes a long time.

The lazy_static dependency would probably need to be added to be able to check the CLI configuration easily from multiple sites, since it allows a 1-time initialized singleton pattern.

Let me know if this is OK with you, and I can start to work on it.

[vrruiz]

I agree. As features are added, using a proper option parser will become important.

[vrruiz]

I'm going to merge this, but ideally xisfreader.rs and fitswriter.rs shouldn't depend on CLI to enable the verbose mode. The idea is to use them like libraries.

[Razican]

That makes sense. Maybe they should be libraries then? We could do a small workspace to have 1/2 libraries and one executable

[vrruiz]

I didn't know what workspaces are. Now I do ;) Yeah, that solution is interesting.

[Razican]

In that regard, maybe it makes sense to change the println!() code to the log crate, and use env_logger to decide what to print.

[vrruiz]

I have partially added log support in xisfreader, to be able to execute tests without calling to CLI.verbose().

[vrruiz]

For debug messages, I removed CLI.verbose() and used env_logger instead.