Reconsider requiring the data to have specific column names including location in get_baseline_predictions

[elray1]

In some sense this is actually 2-3 separate issues, all related to columns required in the input target_ts:

1. Do we need to require that the data has a column containing values of location?
2. What if my data has another "key" or "stratifying" variable, like age_group?
3. Do we need to enforce specific names for these columns like time_index and observation, or could we allow for flexibility in these column names, possibly even via a formula interface