[eigen] initialize vector3d from array

[Nico Schlömer <nico.schloemer@xxxxxxxxx>]

Hi everyone,

I have a large array with the data organized like

```

x0, y0, z0, x1, y1, z1, ...

```

I would like to create some `const Vector3d` from these. How can I do that most efficiently?

Cheers,

Nico