Allow string assignment to atomradiitable.
WIP - OverlapCalculator does not serialize.
Add serialization for AtomRadiiTableWrap.

Also fix ConstantRadiiTable pickling on MacOSX.
ConstantRadiiTable would not serialize when owned by OverlapCalculator.
It seems there would be multiple instantiations of the serialize
archive operators.
Move serialization_to/from_string to libdiffpy DLL.

Adjusted for serialization templates instantiated in the shared library.
Seems to work well for the serialization of AtomRadiiTable.
Do not pickle attributes for a pure C++ class.

ConstantRadiiTable is a wrap of pure C++.  Since the C++ class has
no dictionary attributes, neither should have its Python wrap.
Add explicit wrappers for C++ PDFEnvelope classes.

Also fixed pickling for the Python-wrapped PDF envelope functions.
Dictionary is only pickled for Python-extended classes, it makes
little sense for the C++ classes.