Fix charge convention in Fermi-Hubbard example

[lkdvos]

This PR fixes #200.

The problem being that the charge shift convention changed sign, so the example was trying to contract a PEPS at filling -1 instead of filling 1. Therefore, the norm of the PEPS was actually being correctly computed to be zero, which is an interesting effect since all of the tensors that are involved in the contraction still have norm 1.

In other words, while contracting a pepstensor with itself gives norm 1, as you contract an increasingly large patch the value tends to 0. Definitely an interesting effect!.

I reran all of the examples to update the docs accordingly.

Thanks to @ogauthe for telling me about this!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[pbrehmer]

Looks good, thanks for fixing that and rerunning all the examples!