Impossible to create nested select for namespaced class

[Virviil]

https://github.com/platanus/activeadmin_addons/blob/7908cb41efc85c37c338a872359a0449b14224fb/app/inputs/nested_select_input.rb#L69

Algorithm does not support namespaced models, probably another one attribute should be added, in order to directly specify the class. For example:

instance = level_data[:class_name] || instance_from_attribute_name(level_data[:attribute]) 

Thus, classes like MyNamespace::MyModel can be used