`UnitQuaternion` doesn't guarantee it's a unit quaternion

```rust
use nalgebra as na;

fn main() {
let q = na::UnitQuaternion::from_quaternion(na::Quaternion::new(0.2, 0.3, 0.4, 0.5));
println!("q len {}", q.norm());
let mut a = q.clone();
for _ in 0..50{
a *= a;
}
println!("a len {}", a.norm());
}
```

```sh
q len 0.9999999999999999
a len 0.9930460900296578
```

Which is a huge problem when doing robotics.