Save tree structure

[jordansauce]

Save the distribution of branch norms, and the tree structure of branches over time, including non-sampled branches.

To do:

• Test
• Make plotting functions

Rather than just BranchValues (which contains no information on how each expectation value is positioned within its lineage in the tree), Can you also keep track of and save a tree-structure every synchronization time, which includes nodes for the discarded brances, which come from accepted branch decompsitions where the max_branches budget was too small to support them (they were accepted but not sampled in the # --- Branch Sampling --- phase). This tree structure should have information on the timestep when each branch was created, the wall clock time when it was created, its probability, whether it was sampled, its id, whether it has children, where in the lattice the branching occurred, etc. but not the actual MPS tensors themselves. This tree structure should be kept updated regularly, but only saved every time self.save() is run. Probably the cleanest way to do this is just to use the BranchingMPS class itself as this tree structure, making sure also to create (mostly empty) nodes for non-sampled branches and making sure that each node has all of the requisite info, then just make a function to turn the BranchingMPS class into a stripped down tree without the tensors, which can be saved to disc every time self.save() is run.