A way to detect bad iDMRG convergence stemming from the presence of a domain wall.

[tomohiro-soejima]

This issue is half-way between a documentation improvement and a feature request. The main problem I want to address is the follwing:

Sometimes iDMRG can converge to a state with a domain wall, and this wreaks havoc on the algorithm. Two things that go particularly wrong:

1. E_DMRG and E_MPO will be different. Sometimes the difference is as large as the norm of the Hamiltonian itself!
2. norm_err does not go down even though the state itself is converged.
   I have detailed this observation in this pull request: Add a notebook about how a domain wall can cause bad behavior. tenpy_notebooks#2

With this in mind, my requests are

1. Can you take a look at the notebook, and comment if the content looks appropriate?
2. Can we brainstorm about ways to detect the behavior coming from a domain wall?
   a. I can imagine reporting the difference between E_DMRG and E_MPO and emitting a warning might be a good way to start (although I understand computing E_MPO is often prohibitively expensive).