Explicitly error out when module get loaded twice and the old one is used after

[ninjapapa]

As discovered in #1392, there is a potential issue on Python 2.7 of calling TX.load and run an output module after call TX.allDataSets may fail on requiresDS (as return [None] ).

This issue is to

1. Create a unittest to reproduce the issue, and
2. To find a way to solve it

[ninjapapa]

Issue was reproduces:

        m = self.smvApp.dsm.load('mod:stage5.modules.EmploymentByState')[0]
        allDs = self.smvApp.dsm.allDataSets()
        self.smvApp._module_rdd(m, collector)

Basically if a module is "loaded" then load again, the first loaded version will not be able to rdd on.

[ninjapapa]

It is an expected behavior. However to help future development, we can

• Improve the error message to explicitly point out that call load the second time discard the first time result
• Make sure Python 3.6 also fails

[ninjapapa]

Changed the title to reflect what to be done.