Generalize MPS addition to include MPO*MPS expressions

A neat generalization of ITensor/ITensors.jl#528 is to add up any objects that we can form a density matrix for. So it would be pretty straightfoward to add up arbitrary mixtures of MPS and MPO*MPS, all in a single call, for example (basically by merging the density matrix MPO*MPS contraction algorithm with the new density matrix MPS addition algorithm). We could use a notation like the following:

ϕ = ψ₁ + ψ₂ + (H, ψ₃)

where you can think of (H, ψ₃) as a lazily contracted MPS and MPO, and perhaps we could even move towards H * ψ being a lazy contraction for MPO and MPS. This could also work for adding up MPOs and products of MPOs.