Introduces fast declaration style for factory

Created by: stefanoborini

Makes it faster to define the factory by specifying just the class, factoring out the boilerplate of the reimplementation of the method.

This is backward compatible with the current plugins

force_bdss/data_sources/i_data_source_factory.py

 from envisage.api import Plugin
 from traits.api import Interface, String, Instance, Type
 
-from force_bdss.data_sources.base_data_source import BaseDataSource
-from force_bdss.data_sources.base_data_source_model import BaseDataSourceModel
-
 
 class IDataSourceFactory(Interface):
     """Envisage required interface for the BaseDataSourceFactory.
@@ -15,9 +12,13 @@ class IDataSourceFactory(Interface):
 
     name = String()
 
-    data_source_class = Type(BaseDataSource)
+    data_source_class = Type(
+        "force_bdss.data_sources.base_data_source.BaseDataSource"
+    )
 
-    model_class = Type(BaseDataSourceModel)
+    model_class = Type(
+        "force_bdss.data_sources.base_data_source_model.BaseDataSourceModel"
+    )
 
     plugin = Instance(Plugin)