Fill docstrings for PeakWidthModel classes.

Also add property wrappers for the ConstantPeakWidth and JeongPeakWidth
classes.
Decorate function end with return.

No change in code function.
Fix for use of non-constant StructureAdapterPtr.

This reverts commit 85d08d66351f7d7bf6f97eaf887d508515264574
and 78c7d72733dd47f0298304f790e6caaf034b07a4.
Use shared helpers _pickle_getstate, _pickle_setstate.

No need to define the same code several times.
Improve PDFBaseline pickling.

Wrappers to C++ classes should not pickle wrapper __dict__,
it has no use.
Add pickling support for PeakWidthModel class.