Consider being compatible with Dimensioned

See the suggestion of @paholg [there](https://github.com/sebcrozet/nalgebra/issues/207#issuecomment-257046234).

We have to investigate what consequences this would have on what the user could be able to do with vectors or matrices parametrized by dimensioned numbers. For example:
- How does this integrates with multiplicative algebraic structures?
- How usable is this for operators which, e.g., multiplies all its components together:

``` rust
impl Vector6 {
fn multiply_all(&self) -> N {
let mut result = 1;
for i in 0 .. self.dimension() {
result *= self.component(i)
}
result
}
}
```

With dimensioned components, we could not write the loop anymore, and it wouldn't be implementable for dynamically-sized vectors.