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.

doc/source/api/force_bdss.io.rst

+force_bdss.io package
+=====================
+
+Subpackages
+-----------
+
+.. toctree::
+
+    force_bdss.io.tests
+
+Submodules
+----------
+
+force_bdss.io.workflow_reader module
+------------------------------------
+
+.. automodule:: force_bdss.io.workflow_reader
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
+force_bdss.io.workflow_writer module
+------------------------------------
+
+.. automodule:: force_bdss.io.workflow_writer
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
+
+Module contents
+---------------
+
+.. automodule:: force_bdss.io
+    :members:
+    :undoc-members:
+    :show-inheritance: