Could you encapsulate this memcpy as something like:

```
template <typename T>
inline void fast_copy(const Matrix<T,Dynamic,1>& v_from, Matrix<T,Dynamic,1> v_to) {
    v_to.resize(v_from.size());
    std::memcopy(&v_from(0), &v_to(0), v_from.size() * sizeof(double));
}
```

Then you could drop the mInv(z.mInv.size()) in the ctor.  But there could also be an already sized version.  I don't think we need error checking here as it's an internal function.