Add 2016, 2017, 2018 CPS Support

[andersonfrailey]

This PR modifies the CPS scripts to support using the 2016, 2017, and 2018 March supplements and makes it generally possible to not use years where we don't have C-TAM imputations yet. Some things to note:

• Values for Medicaid, Medicare, WIC, and (for 2016 and beyond) housing benefits are not included in the CPS so the final datasets produced without C-TAM imputations do not either. As such, when we distribute other_ben, the only variable we do it by is SSI.
• I modified the TaxUnit creation process to work without C-TAM benefits. This did not cause any change in the final file produced when using the 2013, 2014, and 2015 supplements with C-TAM imputations.
• While working on this, I noticed that in 2016 and 2017 there is no business income (e00900) in the highest income bin we use for the adjustments in finalprep.py. That seems odd to me and worth investigating at some point. But the result of this is our adjustment method fails to keep the weighted sum of e00900 unchanged. My solution for now is to simply reverse the adjustment. Eventually we should look into changing the methodology or determining why we see no business income in those bins.
• I didn't change the stage 2 scripts at all since we're sticking with 2013-15 as the main files. I'll add some documentation for others on how to update the stage 2 scripts if they want to use a different year.

[andersonfrailey]

If there are no objections, I'll merge this in on Monday.

[andersonfrailey]

EDIT:

On second, second thought, there are actually quiet a few changes in this PR I'll want to keep even after issue #342 is addressed. So, I'm still going to merge this now and then get the new PR open.