Refactor include_columns and exclude_columns
remove n_data_cols
Make more use of colnames property
Remove table.cols, unneeded comments, unneeded DictLikeNmy
remove index for columns - since all columns are always there, no separate index is needed
remove leftover comment
change the way null values are handled in IPAC table writing

fixes #1344
add strip to remove extra whitespace in python3
add missing import statement
simplify new test to not to depend on votable and its unicode handling
remove try-except that could mask other errors
Allow tuples to be passed to is_equivalent
Added note to changelog
Added a few negative assertions to test
Don't need to use an explicit list
Implement ``__eq__`` for Column objects so we can guarantee the behavior
Change interpretation of lists in Table.__getitem__ (closes #274)
Duck type when item in __getitem__ is not one of a few specified possibilities
Add tests and documentation for getting sub-set of columns with list of names
Let `string * Unit` return a `Unit` instead of a `Quantity`