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

@@ -4,7 +4,7 @@ from .i_data_source_bundle import IDataSourceBundle
 
 
 class BaseDataSourceModel(ABCHasStrictTraits):
-    bundle = Instance(IDataSourceBundle)
+    bundle = Instance(IDataSourceBundle, visible=False, transient=True)
 
     def __init__(self, bundle, *args, **kwargs):
         self.bundle = bundle