Compute elementary differentials

[ketch]

This is required in order to compute a full B-series for an integrator applied to a specific problem. This involves computing the tensors of partial derivatives of a function, and then taking certain contractions of those tensors. We want to construct the contractions in code, as is done in bseries.py (the alternative would be writing/generating hard code for every tree). I haven't found a tensor library in Julia that can do this.