Add method setAtomRadiiTableByType.

Also switch to default serialization for AtomRadiiTablePtr.
Use unique instances of serialization functions in DLL.

Implement serialization_tostring and serialization_fromstring helpers
that can be used for pickling in the diffpy.srreal wrap.
All relevant classes should have explicitly instantiated templates
for iarchive, oarchive from diffpy::serialization.
Clean serialization comments.

No change in code function.