Use unserialize() for models in models.activity
Make PumpObject.unserialize usable by subclasses
Remove .unserialize() from subclasses

We are removing .unserialize() from subclasses only where
the method is identical to PumpObject.unserialize()
Unclutter PyPump class by removing Pump.Activity

We don't need a shortcut to create this object, it's usually only
created when unserializing.