Raise runtime exception instead of cassert abort.

Invalid or missing files should just raise exception.
cassert should be used for code logic.
Avoid unused variable warning.

No change in code function.
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.
Use pointer-serialization macro.
Merge serialization from branch 'develop' into ublas.