WindowMPO

[sanderdemeyer]

Currently, WindowMPOHamiltonian is defined, but not its dense equivalent WindowMPO. As a consequence, I could not run DMRG on WindowMPO * WindowMPS (with e.g. the WindowArray), because the corresponding environments are not defined. Looking at the code, I think most of the functionality should be covered by extending the definitions in src/operators/windowhamiltonian.jl and the environment function of a WindowMPOHamiltonian in src/environments/finite_envs.jl. Hence this discussion on the best way to do this. Either we just define a new type, or we define some new struct that covers both WindowMPOHamiltonian and WindowMPO. But any other suggestions are of course welcome.

I would also like to call dot(WindowMPS, WindowMPO, WindowMPS), which would reduce to a dot(FiniteMPS, FiniteMPO, FiniteMPS), but here again only its equivalent dot(FiniteMPS, FiniteMPOHamiltonian, FiniteMPS) is defined. Extending this might be a bit more work?