WIP: PIMixin: Support not exporting initial state

The results at the initial time can contain bogus values for (algebraic) states that do not have their initial state specified. Note that not specifying an initial state for algebraic states may be desired to avoid infeasibilities. When running a closed loop hindcast with FEWS, these bogus values mean that the user will have to do a transformation for every imported time series to cut off the first value (lest it overwites a correct existing value).

This commit makes that process much easier, by allowing users to set a flag to make PIMixin skip writing the results at the initial time.

Closes #1049