§ Chain rule functorially

Let's think through the functoriality. It says that if we have two arrows which can be composed, (f,a)(f, a) and (g,f(a))(g, f(a)) -- note that it has to be (g,f(a))(g, f(a)) because only compatible basepoint functions can be composed. I feel like we shouldn't use natural numbers for MM, but we should rather use real vector spaces. And as for EucEuc, we should replace this with ManOpenManOpen where we use based "charted" opens of a differentiable manifold. This makes the diffgeo clear!