Checkpointing work for selecting bkgd points. Refs #9708.
Refs #9708.  Solved the issue.

and improved documentation.  Added a new test data in
AutoTestData/UsageData.
Tried to fix doc build. Refs #9708.
Tried to fix 2 failures. Refs #9708.
Tried to fix the last failure. Refs #9708.
Hopefully fixed the error. Refs #9708.
Merge remote branch 'origin/feature/9708_processbkgd_python'