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?

src/icartt/dataset.py

@@ -113,7 +113,7 @@ class DataStore1001:
 
 
 class DataStore2110(collections.UserDict):
-    """Data model for FFI1001"""
+    """Data model for FFI2110"""
 
     def __init__(self, ivar, ibvar, auxvars, dvars):
         self.ivarname = ivar.shortname