Qpmnguyen time agg vignette

[ryantibs]

This is me working off of Quang's #37.

[ryantibs]

@qpmnguyen great job. I passed over and made edits here and there. Apart from condensing and rearranging text/code blocks, a brief summary:

• Switched to Massachusetts and Vermont to demonstrate that if you use county name as the key, it'll fail, since they have overlapping county names
• Induced missingness by censoring according to small values, to make the point that zero-filling can be important for downstream stuff like trailing averages