Instantiate serialization for PeakWidthModelPtr.
Revert "Use StructureAdapterConstPtr in PairQuantity."

This reverts commit d6ebd8e7f1ff3d0169fa85cf6702d614e8434224.
StructureAdapterConstPtr was causing all kinds of grief in
Python wrapper classes.  We shall cache structure data in PQEvaluator
so there is no problem to have non-constant StructureAdapter
in PairQuantity.