Warning: is_writeable is deprecated since 0.6.3. Use PersonaStore.is_primary_store.
Whether the PersonaStore is writeable.
Only if a PersonaStore is writeable will its Personas be updated by changes to the Individuals containing them, and those changes then be written out to the relevant backing store.
If this property is false, it doesn't mean that Personas in this 
            persona store aren't writeable at all. If their properties are updated through the 
              Persona, rather than through the Individual containing that persona, changes may 
            be propagated to the backing store.
PersonaStores must not set this property themselves; it will be set as appropriate by the IndividualAggregator.
0.1.13