Reuse Projected Hamiltonian Tensors

It occurs to me the current setup of using TDVP has a major inefficiency: after calling the tdvp function on a given timestep, for the next timestep the exact same MPS that was returned is often reused as the starting state for the next timestep. This means that many of the "environment" tensors representing the Hamiltonian projected into the MPS basis are being recomputed two times when they could be computed just once.

Some solutions could be:

• enable saving the projection tensors and passing them back to the algorithm each time
• making a second, higher-level interface where one specifies the total time value and then that code runs TDVP where a certain amount of time evolution is done each nsweeps sweeps

Start with the tdvp function and trace how the projected Hamiltonian environment tensors are built across successive timesteps. Compare the two proposed approaches in the issue, and consider the work complete when a returned MPS can be reused without recomputing identical projection tensors.