[WIP] Add button to allow CPS as input data source

[GoFroggyRun]

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[GoFroggyRun]

@hdoupe this is how I'd like to add the CPS button. I'm having trouble to have those options displayed properly, even if for most part I was mimicking the "Start Year" button.

[GoFroggyRun]

I'm confused why the new button shows nothing: when I substitute start_years for data_sources, and start_year for default_source, the drop down does show different years as specified in constants.py. But I can't quite figure out why data_sources, default_source won't work.

[hdoupe]

@GoFroggyRun this makes sense to me. In PR #789, I tried to mimic the start year behavior and did not have much success either. Would you mind fixing the merge conflict so that I can check this branch out locally and tinker with it some?

[GoFroggyRun]

@hdoupe Thanks! Conflicts are resolved.

[hdoupe]

Thanks @GoFroggyRun

[GoFroggyRun]

Closing this in favor of #835