Update cdc_baseline_forecaster to handle 0 in aheads, default to aheads = 1:4

[brookslogan]

In order to reproduce & make default the changes made Nov 1 here + enable simplification of this baseline generation script.

[dajmcdon]

@brookslogan To be clear, is the only change that aheads = 1:4 rather than 1:5? nsims ==1e5 already. I'm going to add this to #271 (will come with a version bump to 0.0.7).

[dajmcdon]

Other notes:

• location_to_abbr() function in the script is in epipredict but not exported.
• Any suggestions for how to automatically handle the forecast_date / reference_date mismatch?
• The formatting beginning on line 231, can any of that be added to the *_formatter()?