Add a simple fermionic optimization transpiler pass

[mrossinek]

What should we add?

The qiskit_fermions.transpiler module introduced in #53 suggests the existence of a fermionic-level optimization transpilation stage. During this stage, certain circuit optimizations can be performed while it is still defined in the fermionic (rather than qubit) basis.

We should add at least one example of such a use case (and implementation) to showcase the flexibility and power that this allows for.