This makes the old cross-product more efficient. The old version
initialized a FieldVector with zeros and then filled it. The
new version builds it's directly using consise bracket notation.
The result can be moved without creating a copy.

Implement a cross product in 2d that returns a scalar.
Not mathematically clean but still useful.

2d and 3d cross products now explicitly only work in 2d and 3d.
Other dimensions will trigger a compiler error.