Created get_col method with tests and tutorial.rst update

Useful when you have multiple columns with the same header

docs/tutorial.rst

@@ -146,6 +146,13 @@ To do so, we access the :class:`Dataset` as if it were a standard Python diction
     >>> data['First Name']
     ['Kenneth', 'Bessie']
 
+You can also access the column using its index. ::
+
+    >>> d.headers
+    ['Last Name', 'First Name', 'Age']
+    >>> d.get_col(1)
+    ['Kenneth', 'Bessie']
+
 Let's find the average age. ::
 
     >>> ages = data['Age']