Sweeping algorithms for tree tensor networks

[leburgel]

Moved here from ITensor/ITensorTDVP.jl#43

A first working attempt at generalizing MPS sweeping algorithms to tensor networks on arbitrary tree geometries. All algorithms should work for both ITensors.MPS and ITensorNetworks.TreeTensorNetworksState (except some of the newer additions which I haven't generalized yet), with the same syntax currently used in ITensorTDVP.jl.

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.

[mtfishman]

Thanks @leburgel, this is great. It is a big step forward for ITensor, and will become our standard DMRG/TDVP code!