Fix memory leak in Python binding of istore_get_uuids()

[jesper-friis]

The C memory returned by dlite_swig_istore_get_uuids() will be leaked when the wrapped Python object goes out of scope.

We need to tell SWIG that is has ownership of the memory and that it should free it when the corresponding python object goes out of scope. Can be done with the SWIG %newobject and %delobject directives.