Safer plugin import

Created by: stefanoborini

First implementation and change of API to make the plugin import system more robust against broken plugins.

This PR requires we readapt all plugins. Similar changes are required also for the other classes API so that it's uniform.

force_bdss/tests/probe_classes/probe_extension_plugin.py

+from force_bdss.base_extension_plugin import BaseExtensionPlugin
+from force_bdss.tests.probe_classes.data_source import ProbeDataSourceFactory
+from force_bdss.tests.probe_classes.mco import ProbeMCOFactory
+from force_bdss.tests.probe_classes.notification_listener import \
+    ProbeNotificationListenerFactory
+from force_bdss.tests.probe_classes.ui_hooks import ProbeUIHooksFactory
+
+
+class ProbeExtensionPlugin(BaseExtensionPlugin):
+    def get_producer(self):
+        return "enthought"
+
+    def get_identifier(self):
+        return "test"
+
+    def get_factory_classes(self):
+        return [
+            ProbeDataSourceFactory,
+            ProbeMCOFactory,
+            ProbeNotificationListenerFactory,
+            ProbeUIHooksFactory,
+        ]