dimension and unit system for general physical physical quantities.
```rust use ::physical_quantity::*;
let inch: Unit = "in.".parse().unwrap(); let pq = inch.pq(1f64); let mm: Unit = "mm".parse().unwrap(); let inch2mm = mm.value(pq).unwrap();
assert_eq!(inch2mm, 25.4); ```
This package is in the very early stage.