Serialization/Deserialization of derived variable catalog

[andersy005]

#379 introduced "variable derivation" via a registry of decorated functions. One of the missing features of the derived variable catalog is serialization/deserialization.

[andersy005]

#386 added functionality for loading a registry from a Python module and this seems to suffice for the time being. The serialization feature doesn't seem to be necessary. We can revisit the serialization of a registry to a Python module at a later time.