We used to have Jordan-Wigner and Bravyi-Kitaev implemented in the old xacc-vqe. When we moved we only ported JW.

We need to build out the ObservableTransform with support for other fermion to spin transformations, or even the reverse.

Let's start by adding BK back, we can follow the impl in https://github.com/ORNL-QCI/xacc-vqe/blob/master/transformations/bk/BravyiKitaevIRTransformation.cpp

Then we need to go through and enable users to change the transform from JW to what they want (like with VQE, etc)