Sweeping algorithms for tree tensor networks

[leburgel]

Work in progress, a first working attempt at generalizing MPS sweeping algorithms to tensor networks on arbitrary tree geometries as defined in ITensorNetworks.jl. This relies on ITensor/ITensorNetworks.jl#18, which is also still a work in progress. All algorithms should just work with the current syntax for both ITensors.MPS and ITensorNetworks.TreeTensorNetworksState (except some of the newer additions which I haven't generalized yet).

Aside from some patching to work with MPS and trees using a common interface, the main internal changes are how the sequence of update steps within an individual sweep is generated and how individual local updates are performed. The idea is that a sequence of SweepSteps is generated for the given geometry depending on the kind of sweeping being used (e.g. 1- or 2-site, reverse steps or not), where each SweepStep contains all the information needed for a single local update. This allows to split off the geometry-dependent part and adds flexibility in the sweeping sequences used. In addition, by treating each local update step in the same way we can use a single update routine for all local updates (whether it's a bond- or site-update, forwards or backwards evolution...) irrespective of what happens before or after in the sequence.

There's still quite a few things to do, and I will keep updating this whenever I find the time.

[mtfishman]

@leburgel this would be ready to move directly to https://github.com/mtfishman/ITensorNetworks.jl once ITensor/ITensorNetworks.jl#43 is merged.

[mtfishman]

@leburgel I merged ITensor/ITensorNetworks.jl#43 so this can be moved to that repository.

[leburgel]

Moved to ITensor/ITensorNetworks.jl#44.