Adds _append_column method to Corpus class.
Changes compare_items method to default return
a list of bools; True: item with that index in _dict exists
in the comparison set.

Fixes show_missing_items to display correct subset.
Changes has_value method to default return
a list of bools; True for items that satisfy the expression
in the input args.
Adds copy_columns method. Copies specified columns
from another Corpus instance or dictionary-like object.
Values not found in the comparison corpus are entered
as 'None'.
Adds copy_columns method. Copies specified columns
from another Corpus instance or dictionary-like object.
Values not found in the comparison corpus are entered
as 'None'.
Moves copy_items to Corpus class.