Empty namespace that plugins should use to store themselves, so they can be
loaded via Model.plugin.
Plugins should be modules with one of the
following conditions:
- A singleton method named apply, which takes a model and additional
arguments.
- A module inside the plugin module named InstanceMethods, which will be
included in the model class.
- A module inside the plugin module named ClassMethods, which will extend the
model class.
- A module inside the plugin module named DatasetMethods, which will extend
the model‘s dataset.