`ITensors.MPS`/`ITensors.MPO` graph interface layer

Move the ITensors.MPS/ITensors.MPO graph interface layer from https://github.com/ITensor/ITensorTDVP.jl/pull/43 into this package, once that PR is merged.

An interesting question that hits on some limitations of Julia's subtyping system is that it would be nice for ITensors.MPS/ITensors.MPO to work in the functions defined in these packages, once they adhere to the AbstractDataGraph interface. However, it is awkward to try to make them subtypes of AbstractDataGraph.

It's possible to do that without making ITensors.jl depend on the full DataGraphs.jl package by making an interface package DataGraphsCore.jl in the spirit of StaticArraysCore.jl but that seems a bit overkill. The alternative is to use traits implemented with the SimpleTraits.jl package, and define traits such as IsITensorNetwork or IsDataGraph which are then overloaded for ITensors.MPS/ITensors.MPO, but that also seems a bit overkill as well since then we would need to define all of the functions we currently define through abstract type dispatch with AbstractITensorNetwork or AbstractDataGraph with traits instead.

In the end, it may not matter so much since I think the ITensorNetworks.TreeTensorNetwork type will supersede the ITensors.MPS/ITensors.MPO types but be superior in a variety of ways, so we can just keep some simple interfacing functionality and have conversions from ITensors.MPS/ITensors.MPO to ITensorNetworks.TreeTensorNetwork.

Start by reviewing ITensorTDVP.jl pull request #43 and the AbstractDataGraph interface discussed in the issue, after confirming that PR is merged. Compare the proposed MPS/MPO interfacing options and determine which simple functionality and conversions to ITensorNetworks.TreeTensorNetwork are still needed; done means the relevant functions accept MPS/MPO as described.