Use humantime crate in main

[Luthaf]

We currently manually format the elapsed time manually in the output log:

lumol/src/bin/lumol.rs

Lines 66 to 87 in 0f6efbd

	fn format_elapsed(elapsed: Duration) -> String {
	if elapsed.num_weeks() > 0 {
	let h = elapsed.num_hours() % 24;
	let d = elapsed.num_days() % 7;
	let w = elapsed.num_weeks();
	format!("{} weeks {} days {}h", w, d, h)
	} else if elapsed.num_days() > 0 {
	let m = elapsed.num_minutes() % 60;
	let h = elapsed.num_hours() % 24;
	let d = elapsed.num_days();
	format!("{} days {}h {}min", d, h, m)
	} else if elapsed.num_hours() > 0 {
	let s = elapsed.num_seconds() % 60;
	let m = elapsed.num_minutes() % 60;
	let h = elapsed.num_hours();
	format!("{}h {}min {}s", h, m, s)
	} else {
	let s = elapsed.num_seconds() % 60;
	let m = elapsed.num_minutes();
	format!("{}min {}s", m, s)
	}
	}

This code could be replaced by using the humantime crate instead.