[examples] Differentiable types should not require vector space structure, only tangent types

[juniorrojas]

Some examples that use differentiable types also require an underlying vector space for the differentiable type. However, this should not be necessary. Only the tangent type associated with a differentiable type needs a vector space. In other words, we need definitions for tangent + tangent, scalar * tangent and primal + tangent, but primal + primal and scalar * primal should not be necessary.