FEATURE: Revisit present_agg_severity argument in agg

[chacalle]

Is your feature request related to a problem? Please describe.

These are the current options for present_agg_severity

#' **`present_agg_severity`** (`agg` only):
#'
#' Check for requested aggregates in `mapping` that are already present
#' 1. `stop`: throw error (this is the default).
#' 2. `warning` or `message`: throw warning/message, drop aggregates and continue
#' with aggregation.
#' 3. `none`: don't throw error or warning, drop aggregates and continue with
#' aggregation.
#' 4. `skip`: skip this check and add to the values already present for the
#' aggregates.

For 2/3 we are considering whether there should also be the option to preserve the original aggregate values rather than only the option to drop and re-aggregate.

Describe the solution you'd like
Add the option to either drop/re-aggregate or preserve the original aggregate.

If not easy to add both options, then default to preserving the existing aggregate.

Additional context
Originally brought up here https://github.com/ihmeuw-demographics/hierarchyUtils/pull/54/files#r538626632