Fix for zones reopening in simulation-based constraint mechanism for work/school location choice

[dhensle]

Fix for #820: If zones close, they now remain closed.

[jpn--]

This is great! Here's why I like it:

• You have written a clear and simple test that creates a tiny model, runs it, and fails on the existing code in a manner that clearly demonstrates the existing problem and runs in under one second.
• After the code changes are made, the test passes. Voila!

The one thing that's missing is a note on the change log activitysim/docs/dev-guide/changes.md. This is not really what may of us would call a "breaking change" as it should not formally cause any existing model to fail, but it does break strict reproducibility between versions. This is seen in the SANDAG test, which now fails because the shadow-priced location choices are changing just a tiny bit.

[dhensle]

Great! Glad you like the test -- it was not a trivial exercise to get a nice CI test built for this particular issue. And we also now have a CI test for the simulation-based constraint mechanism as a whole as a bonus!

I have updated the change log. I think this is ready to merge!