fixed issue where CSVTabularData failed, and added supporting tests

I encountered an issue where griddle did not work for CSV files because the RowIterator constructor on line 39 of CSVTabularData expected a List, but for a Collection due to the getter method on line 27. I updated the getter to return Iterable to match the overridden signature, and used the Groovy syntax to bypass the getter and access the field directly. I also added tests to ensure that CSV reading works as expected, based on the existing tests for Excel files.