Extract io layer to writer/reader class

Created by: stefanoborini

Extracted IO layer into writer and reader classes, and create model objects with support from the bundles.

force_bdss/data_sources/base_data_source_model.py

-import abc
-from traits.api import ABCHasStrictTraits
+from traits.api import ABCHasStrictTraits, Instance
+
+from .i_data_source_bundle import IDataSourceBundle
 
 
 class BaseDataSourceModel(ABCHasStrictTraits):
-    @classmethod
-    @abc.abstractmethod
-    def from_json(self, model_data):
-        pass
+    """Base class for the bundle specific DataSource models.
+    This model will also provide, through traits/traitsui magic the View
+    that will appear in the workflow manager UI.
+
+    In your bundle definition, your bundle-specific model must reimplement
+    this class.
+    """
+    #: A reference to the creating bundle, so that we can
+    #: retrieve it as the originating factory.
+    bundle = Instance(IDataSourceBundle, visible=False, transient=True)
+
+    def __init__(self, bundle, *args, **kwargs):
+        self.bundle = bundle
+        super(BaseDataSourceModel, self).__init__(*args, **kwargs)