Add Workflow and Slot to the api

Created by: martinRenou

Fixes #90 (closed)

force_bdss/api.py

 from .base_extension_plugin import BaseExtensionPlugin  # noqa
 from .ids import factory_id, plugin_id  # noqa
+
 from .core.data_value import DataValue  # noqa
+from .core.workflow import Workflow  # noqa
+from .core.slot import Slot  # noqa
 
 from .data_sources.base_data_source_model import BaseDataSourceModel  # noqa
 from .data_sources.base_data_source import BaseDataSource  # noqa