Hubbard model notebook is flaky

[fdmalone]

From #869 we see some regression with FQE in the sense that the Taylor series might not converge. It's not clear when this broke or if it's a flaky numerical thing (too tight a threshold). There are a few options:

1. Update FQE to accept a threshold
2. Figure out if this is flaky or some external library update somewhere which lead to worsened precision.

At the very least we should test OUR notebooks where possible as part of some nightly tests.

[mhucka]

@fdmalone I may have asked you about this before – sorry if so. Since you assigned yourself this issue, can you let us know the status and your plans?

[fdmalone]

I won't address this within the next ~ two months

[mhucka]

@fdmalone Thanks for your reply. OK, let's keep it open. I assigned it a P2 – does that sound right?