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
Merge pull request #1350 from hamogu/implement1350

Refactor io.ascii include_names and exclude_names