Metric spaces

cgmath currently has a [`MetricSpace`](https://docs.rs/cgmath/0.15.0/cgmath/trait.MetricSpace.html) trait that is defined for vectors, points, and quaternions:

```rust
pub trait MetricSpace: Sized {
type Metric: BaseFloat;
fn distance2(self, other: Self) -> Self::Metric;

fn distance(self, other: Self) -> Self::Metric { ... }
}
```

Could we consider rejiggering the `alga::linear` trait hierarchy to add something like this?