Primitive hardness helpers.

Hardness values are only compared when they share the same scale. No approximate cross-scale conversions are provided in this first pass.

Examples

use use_hardness::{Hardness, HardnessScale, is_harder_than, same_scale};

let first = Hardness::new(200.0, HardnessScale::Vickers).unwrap();
let second = Hardness::new(180.0, HardnessScale::Vickers).unwrap();

assert!(same_scale(first, second));
assert!(is_harder_than(first, second).unwrap());