WIP symbolic jacobians

Created by: maedoc

symbolic jacobians are in reach for most of our models, especially with the upcoming DSL. this is a first sketch which computes derivatives symbolically with sympy for Jansen Rit and generates a sequences of instructions which can be strung together, eval'd and then jit'd with numba.

one could then also specialize when parameter values are known by substituting the values in prior to doing the symbolics, which will help as well.