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
Implement ``__eq__`` for Column objects so we can guarantee the behavior
Let `string * Unit` return a `Unit` instead of a `Quantity`
Add test for string * unit and string / unit (and vice versa)
Ensure division is OK too, adapt tests, add line indicating API change
Fixed typo
Preserve mask when comparing masked arrays/columns, and fix column test.
fix errors that appeard after rebasing
Added entry to CHANGES.rst
__array_priority__ isn't needed, and properly test both argument orders for equality.