Introduces base class for plugins to simplify export

Created by: stefanoborini

Introduced a base class BaseExtensionPlugin. This class provides the base class for all plugins that are externally provided, and allows the user to just define the default methods to return their own bundles, instead of having to deal with the extension point ids.

force_bdss/core_plugins/csv_extractor/csv_extractor_plugin.py

-from envisage.plugin import Plugin
-from traits.api import List
-
-from force_bdss.data_sources.i_data_source_bundle import IDataSourceBundle
+from force_bdss.base_extension_plugin import BaseExtensionPlugin
 
 from .csv_extractor.csv_extractor_bundle import CSVExtractorBundle
 
 
-class CSVExtractorPlugin(Plugin):
-    data_sources = List(
-        IDataSourceBundle,
-        contributes_to='force.bdss.data_sources.bundles'
-    )
-
-    def _data_sources_default(self):
+class CSVExtractorPlugin(BaseExtensionPlugin):
+    def _data_source_bundles_default(self):
         return [CSVExtractorBundle()]