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/test_base_extension_plugin.py

+import unittest
+
+from force_bdss.tests.probe_classes.probe_extension_plugin import \
+    ProbeExtensionPlugin
+
+
+class TestBaseExtensionPlugin(unittest.TestCase):
+    def test_basic_init(self):
+        plugin = ProbeExtensionPlugin()
+        self.assertEqual(len(plugin.data_source_factories), 1)
+        self.assertEqual(len(plugin.notification_listener_factories), 1)
+        self.assertEqual(len(plugin.mco_factories), 1)
+        self.assertEqual(len(plugin.ui_hooks_factories), 1)
+        self.assertFalse(plugin.broken)
+        self.assertEqual(plugin.error, "")