Add docs, tests for data interface

Documenting how to add data to ICARTT file in docs and usage_examples. @FObersteiner could you review this and report back if this is sufficient and makes sense to users?

docs/index.rst

@@ -37,6 +37,17 @@ Dataset
 .. autoclass:: Dataset
    :members:
 
+DataStore
+*********
+
+The .data attribute of an ``icartt.Dataset`` is a DataStore, which can be accessed to add data as follows:
+
+.. autoclass:: DataStore1001
+   :members:
+
+.. autoclass:: DataStore2110
+   :members:
+
 Formats
 ********