Status of expr_dev branch

[Bill Hart]

Here's a quick update, since I know that some of you are waiting on this merge:

1. I'm fighting with an intermittent test failure on python3.5.  IMHO, this shouldn't hold-up the merge, but it's disturbing.

2. John is working to resolve GDP logic that I mucked up while trying to merge from master

3. I've gone through my notes from our previous meetings, and I've updated the online docs.  This includes moving code examples into tests.  I've reviewed the wording of the narrative, and I'm pretty happy overall.  If folks see additional issues, they should either edit the branch or add a ticket.

4. I've added tickets for outstanding design issues and related concerns that shouldn't be blockers for the release.  The most worrisome of these is that there are clear issues with the replacement walker.  But I think I shouldn't try to resolve that until after the merge.

I think that (2) is the only thing blocking the merge.

--Bill