Add latency&revision gotchas, pseudoprospective forecasting, backcasting vignette/chapter(s)

[brookslogan]

• Overview, and why to use epix_slide()
• Latency issues: most recent time available vs. ref_time_value
• latency issue: mixed latency
• pseudoprospective forecasting
• revision MAPE curve
• version-aware forecasting
  • apply reasonable model to one location x forecast_date x target signal demonstrating systematic biased revisions
  • make model work for more locations, potentially more signals
  • demonstrate, evaluate pseudoprospective version-aware forecasting