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/tests/test_base_data_source_factory.py

 import unittest
+
+from force_bdss.data_sources.tests.test_base_data_source import DummyDataSource
+from force_bdss.data_sources.tests.test_base_data_source_model import \
+    DummyDataSourceModel
+
 try:
     import mock
 except ImportError:
     from unittest import mock
 
+import testfixtures
+
 from envisage.plugin import Plugin
 from force_bdss.data_sources.base_data_source_factory import \
     BaseDataSourceFactory
@@ -21,8 +28,35 @@ class DummyDataSourceFactory(BaseDataSourceFactory):
         pass
 
 
+class DummyDataSourceFactoryFast(BaseDataSourceFactory):
+    id = "foo"
+
+    name = "bar"
+
+    model_class = DummyDataSourceModel
+
+    data_source_class = DummyDataSource
+
+
 class TestBaseDataSourceFactory(unittest.TestCase):
     def test_initialization(self):
         factory = DummyDataSourceFactory(mock.Mock(spec=Plugin))
         self.assertEqual(factory.id, 'foo')
         self.assertEqual(factory.name, 'bar')
+
+    def test_fast_specification(self):
+        factory = DummyDataSourceFactoryFast(mock.Mock(spec=Plugin))
+        self.assertIsInstance(factory.create_data_source(), DummyDataSource)
+        self.assertIsInstance(factory.create_model(), DummyDataSourceModel)
+
+    def test_fast_specification_errors(self):
+        factory = DummyDataSourceFactoryFast(mock.Mock(spec=Plugin))
+        factory.model_class = None
+        factory.data_source_class = None
+
+        with testfixtures.LogCapture():
+            with self.assertRaises(RuntimeError):
+                factory.create_data_source()
+
+            with self.assertRaises(RuntimeError):
+                factory.create_model()