Use type level integers

[SuperFluffy]

At the moment, Integrator::swap_observer provides the ability to change the type of observer used during the integration. As some simulations might be initially run with one set of variables and then extended (for example, by introducing Lyapunov exponents, adding extra rows the to State matrix), the compatibility between observers and the simulation should be checked at the type level by using type level integers for the matrix dimensions/shape.

This will have to wait until type level integers land in rust.