Add latency definitions & visualization vignette/chapter

[brookslogan]

• Show examples of calculating a few different definitions of latency.
  • Fix bug in nadup, na0dup definitions counting zeros/NAs between the second most recent unique nonzero/non-NA value and the most recent nonzero/non-NA value (when both of these exist).
  • Fix bugfix on all-zero/NA/duplicate time series
  • Make code more comprehensible.
• Visualize state-level latency.
• Consider facility-level visualizations.
• Consider incorporating information about expected levels (population, seasonality / recent national/regional levels).
• Consider adding flagging mechanisms.