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.py

@@ -3,6 +3,11 @@ import six
 
 
 class BaseDataSource(six.with_metaclass(abc.ABCMeta)):
+    """Base class for the DataSource, any computational engine/retriever
+    for data.
+
+    Inherit from this class for your specific DataSource.
+    """
     def __init__(self, bundle, application, model):
         self.bundle = bundle
         self.application = application