Orbital (Local) Optimization

It would be useful to be able to perform orbital optimization. With the recent work of @ShuoyangW , we have the gradient (and soon, the Hessian) with respect to orbitals. Being able to wrap PyCI in an orbital optimizer is a good thing.

This basically means that one needs to be able to consider PyCI a function where 1- and 2-electron integrals are read in, and the energy and 1- and 2-electron density matrices come out. This then needs to be wrapped in another function that takes 1- and 2-electron integrals and performs a unitary transformation of them based on the step accepted from an optimizer, as the optimizer expects to receive the energy, gradient, and (perhaps) Hessian as a function of an argument (probably the antiHermitian matrix that generates the unitary transformation).

The issue names no files, tests, or concrete entry points. Start by tracing how PyCI receives one- and two-electron integrals and produces energy and density matrices; the work is complete when those outputs can support an orbital optimizer using accepted unitary-transformation steps and the requested derivatives.