New Data unit tests

[dsweber2]

After the initial batch, here are some more that may be helpful

• confirming that scaled_pop can produce exactly the same result as arx_forecaster
• data source which has blatantly obvious revision errors, like sudden massive spikes returning to a previous lower value
• better version of delay test (hand craft examples of problems, rather than random offsets)
• Which dates predicted values are available for/making sure that preds are on days where we have enough data
• Right now, looks like predictions are only for dates that are included in the input dataset. Might be useful to check behavior when making true forecasts (again, that output dates are what we expect, etc)

(last two from here)

[dsweber2]

re-using the older meta-issue on this #23