Class Marshallable

Known Subclasses:

BuildInformation, Node, Package, Provide, Require, Unmarshallable

Provides support for versioned marshalling and unmarshalling
of objects. Its functionality is a bit intrusive in that it
requires every class that want to take part in this game to derive
from it.

The functionality is based on `pickle`'s __setstate__ and
__getstate__ which is provided by `Marshallable`. Derived classes
must implement two methods, `get_marshalling_data` and
`set_marshalling_data`, to be able to operate meaningfully.

Return marshalling data for my attributes.

To be implemented by derived classes.

The returned marshalling data is a dictionary object that
contains relatively anonymous data which represents the object
to be marshalled. The dictionary is composed by derived
classes and must look as follows:

::

   {
      'generating_class': <class object of generating object>,
      'versions': <versions of contributions>,
      'attributes': <dictionary with direct attributes key/value pairs>,
   }

Called indirectly by `__getstate__`.

(To be documented)